Write metadata to file cache

Then, before a GET, read the metadata and add any etag to the headers and handle 304 in response.

Cargo.lock

@@ -109,6 +109,7 @@ dependencies = [
  "base64 0.13.0",
  "bytes",
  "cbindgen",
+ "chrono",
  "dirs",
  "http-api-problem",
  "log",
@@ -183,6 +184,20 @@ version = "0.1.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b486ce3ccf7ffd79fdeb678eac06a9e6c09fc88d33836340becb8fffe87c5e33"
 
+[[package]]
+name = "chrono"
+version = "0.4.19"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "670ad68c9088c2a963aaa298cb369688cf3f9465ce5e2d4ca10e6e0098a1ce73"
+dependencies = [
+ "libc",
+ "num-integer",
+ "num-traits",
+ "serde",
+ "time",
+ "winapi 0.3.9",
+]
+
 [[package]]
 name = "clap"
 version = "2.33.1"
@@ -742,6 +757,25 @@ dependencies = [
  "winapi 0.3.9",
 ]
 
+[[package]]
+name = "num-integer"
+version = "0.1.44"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d2cc698a63b549a70bc047073d2949cce27cd1c7b0a4a862d08a8031bc2801db"
+dependencies = [
+ "autocfg",
+ "num-traits",
+]
+
+[[package]]
+name = "num-traits"
+version = "0.2.14"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9a64b1ec5cda2586e284722486d802acf1f7dbdc623e2bfc57e65ca1cd099290"
+dependencies = [
+ "autocfg",
+]
+
 [[package]]
 name = "num_cpus"
 version = "1.13.0"

Cargo.toml

@@ -13,6 +13,7 @@ cbindgen = "0.14.4"
 anyhow = "1.0"
 base64 = "0.13"
 bytes = "0.5"
+chrono = { version = "0.4", features = ["serde"] }
 http-api-problem  = "0.17"
 mockito = "0.26.0"
 reqwest = { version = "0.10", features = ["blocking", "json", "gzip"] }

src/cache.rs

@@ -3,7 +3,9 @@ use std::{fs::create_dir_all, fs::File, io::BufReader, io::Write, path::Path, pa
 use anyhow::{Context, Result};
 use base64::{encode_config, URL_SAFE_NO_PAD};
 use bytes::Bytes;
-use serde::Deserialize;
+use chrono::{DateTime, Utc};
+use reqwest::{blocking::Response, header::HeaderMap};
+use serde::{Deserialize, Serialize};
 #[cfg(test)]
 use tempfile::tempfile;
 
@@ -14,6 +16,12 @@ use std::println as info;
 
 use super::API_VERSION;
 
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Metadata {
+    pub etag: Option<String>,
+    pub date: Option<DateTime<Utc>>,
+}
+
 pub fn file_cache_dir(api_url: &str) -> Result<PathBuf> {
     let encoded_url = encode_config(api_url, URL_SAFE_NO_PAD);
     let path = Path::new("Data/SKSE/Plugins/BazaarRealmCache")
@@ -34,6 +42,34 @@ pub fn update_file_cache(cache_path: &Path, bytes: &Bytes) -> Result<()> {
     Ok(())
 }
 
+pub fn update_metadata_file_cache(cache_path: &Path, headers: &HeaderMap) -> Result<()> {
+    #[cfg(not(test))]
+    let mut file = File::create(cache_path)?;
+    #[cfg(test)]
+    let mut file = tempfile()?;
+
+    let etag = headers
+        .get("etag")
+        .map(|val| val.to_str().unwrap_or("").to_string());
+    let date = headers
+        .get("date")
+        .map(|val| val.to_str().unwrap_or("").parse().unwrap_or(Utc::now()));
+    let metadata = Metadata { etag, date };
+    serde_json::to_writer(file, &metadata)?;
+    Ok(())
+}
+
+pub fn update_file_caches(
+    body_cache_path: &Path,
+    metadata_cache_path: &Path,
+    response: Response,
+) -> Result<Bytes> {
+    update_metadata_file_cache(metadata_cache_path, &response.headers())?;
+    let bytes = response.bytes()?;
+    update_file_cache(body_cache_path, &bytes)?;
+    Ok(bytes)
+}
+
 pub fn from_file_cache<T: for<'de> Deserialize<'de>>(cache_path: &Path) -> Result<T> {
     #[cfg(not(test))]
     let file = File::open(cache_path).context(format!(
@@ -47,3 +83,18 @@ pub fn from_file_cache<T: for<'de> Deserialize<'de>>(cache_path: &Path) -> Resul
     info!("returning value from cache: {:?}", cache_path);
     Ok(serde_json::from_reader(reader)?)
 }
+
+pub fn load_metadata_from_file_cache(cache_path: &Path) -> Result<Metadata> {
+    #[cfg(not(test))]
+    let file = File::open(cache_path).context(format!(
+        "Object not found in API or in cache: {}",
+        cache_path.file_name().unwrap_or_default().to_string_lossy()
+    ))?;
+    #[cfg(test)]
+    let file = tempfile()?; // cache always reads from an empty temp file in cfg(test)
+
+    let reader = BufReader::new(file);
+    info!("returning value from cache: {:?}", cache_path);
+    let metadata: Metadata = serde_json::from_reader(reader)?;
+    Ok(metadata)
+}

src/interior_ref_list.rs

@@ -1,7 +1,7 @@
 use std::{ffi::CStr, ffi::CString, os::raw::c_char, slice};
 
 use anyhow::Result;
-use reqwest::Url;
+use reqwest::{StatusCode, Url};
 use serde::{Deserialize, Serialize};
 
 #[cfg(not(test))]
@@ -10,8 +10,9 @@ use log::{error, info};
 use std::{println as info, println as error};
 
 use crate::{
-    cache::file_cache_dir, cache::from_file_cache, cache::update_file_cache, log_server_error,
-    result::FFIResult,
+    cache::file_cache_dir, cache::from_file_cache, cache::load_metadata_from_file_cache,
+    cache::update_file_cache, cache::update_file_caches, cache::update_metadata_file_cache,
+    log_server_error, result::FFIResult,
 };
 
 #[derive(Serialize, Deserialize, Debug)]
@@ -94,6 +95,7 @@ pub struct RawInteriorRefVec {
     pub cap: usize,
 }
 
+// TODO: delete me if unused
 #[no_mangle]
 pub extern "C" fn create_interior_ref_list(
     api_url: *const c_char,
@@ -133,13 +135,20 @@ pub extern "C" fn create_interior_ref_list(
             .json(&interior_ref_list)
             .send()?;
         info!("create interior_ref_list response from api: {:?}", &resp);
+
+        let cache_dir = file_cache_dir(api_url)?;
+        let headers = resp.headers().clone();
         let bytes = resp.bytes()?;
         let json: InteriorRefList = serde_json::from_slice(&bytes)?;
         if let Some(id) = json.id {
             update_file_cache(
-                &file_cache_dir(api_url)?.join(format!("interior_ref_list_{}.json", id)),
+                &cache_dir.join(format!("interior_ref_list_{}.json", id)),
                 &bytes,
             )?;
+            update_metadata_file_cache(
+                &cache_dir.join(format!("interior_ref_list_{}_metadata.json", id)),
+                &headers,
+            )?;
         }
         Ok(json)
     }
@@ -209,14 +218,13 @@ pub extern "C" fn update_interior_ref_list(
             .json(&interior_ref_list)
             .send()?;
         info!("update interior_ref_list response from api: {:?}", &resp);
-        let bytes = resp.bytes()?;
+
+        let cache_dir = file_cache_dir(api_url)?;
+        let body_cache_path = cache_dir.join(format!("shops_{}_interior_ref_list.json", shop_id));
+        let metadata_cache_path =
+            cache_dir.join(format!("shops_{}_interior_ref_list_metadata.json", shop_id));
+        let bytes = update_file_caches(&body_cache_path, &metadata_cache_path, resp)?;
         let json: InteriorRefList = serde_json::from_slice(&bytes)?;
-        if let Some(_id) = json.id {
-            update_file_cache(
-                &file_cache_dir(api_url)?.join(format!("shops_{}_interior_ref_list.json", shop_id)),
-                &bytes,
-            )?;
-        }
         Ok(json)
     }
 
@@ -245,6 +253,7 @@ pub extern "C" fn update_interior_ref_list(
     }
 }
 
+// TODO: delete me if unused
 #[no_mangle]
 pub extern "C" fn get_interior_ref_list(
     api_url: *const c_char,
@@ -268,25 +277,38 @@ pub extern "C" fn get_interior_ref_list(
         info!("api_url: {:?}", url);
 
         let client = reqwest::blocking::Client::new();
-        let cache_path = file_cache_dir(api_url)?
-            .join(format!("interior_ref_list_{}.json", interior_ref_list_id));
+        let cache_dir = file_cache_dir(api_url)?;
+        let body_cache_path =
+            cache_dir.join(format!("interior_ref_list_{}.json", interior_ref_list_id));
+        let metadata_cache_path = cache_dir.join(format!(
+            "interior_ref_list_{}_metadata.json",
+            interior_ref_list_id
+        ));
+        let mut request = client.get(url).header("Api-Key", api_key);
+        // TODO: load metadata from in-memory LRU cache first before trying to load from file
+        if let Ok(metadata) = load_metadata_from_file_cache(&metadata_cache_path) {
+            if let Some(etag) = metadata.etag {
+                request = request.header("If-None-Match", etag);
+            }
+        }
 
-        match client.get(url).header("Api-Key", api_key).send() {
+        match request.send() {
             Ok(resp) => {
                 info!("get_interior_ref_list response from api: {:?}", &resp);
                 if resp.status().is_success() {
-                    let bytes = resp.bytes()?;
-                    update_file_cache(&cache_path, &bytes)?;
+                    let bytes = update_file_caches(&body_cache_path, &metadata_cache_path, resp)?;
                     let json = serde_json::from_slice(&bytes)?;
                     Ok(json)
+                } else if resp.status() == StatusCode::NOT_MODIFIED {
+                    from_file_cache(&body_cache_path)
                 } else {
                     log_server_error(resp);
-                    from_file_cache(&cache_path)
+                    from_file_cache(&body_cache_path)
                 }
             }
             Err(err) => {
                 error!("get_interior_ref_list api request error: {}", err);
-                from_file_cache(&cache_path)
+                from_file_cache(&body_cache_path)
             }
         }
     }
@@ -355,23 +377,33 @@ pub extern "C" fn get_interior_ref_list_by_shop_id(
         info!("api_url: {:?}", url);
 
         let client = reqwest::blocking::Client::new();
-        let cache_path =
-            file_cache_dir(api_url)?.join(format!("shops_{}_interior_ref_list.json", shop_id));
+        let cache_dir = file_cache_dir(api_url)?;
+        let body_cache_path = cache_dir.join(format!("shops_{}_interior_ref_list.json", shop_id));
+        let metadata_cache_path =
+            cache_dir.join(format!("shops_{}_interior_ref_list_metadata.json", shop_id));
+        let mut request = client.get(url).header("Api-Key", api_key);
+        // TODO: load metadata from in-memory LRU cache first before trying to load from file
+        if let Ok(metadata) = load_metadata_from_file_cache(&metadata_cache_path) {
+            if let Some(etag) = metadata.etag {
+                request = request.header("If-None-Match", etag);
+            }
+        }
 
-        match client.get(url).header("Api-Key", api_key).send() {
+        match request.send() {
             Ok(resp) => {
                 info!(
                     "get_interior_ref_list_by_shop_id response from api: {:?}",
                     &resp
                 );
                 if resp.status().is_success() {
-                    let bytes = resp.bytes()?;
-                    update_file_cache(&cache_path, &bytes)?;
+                    let bytes = update_file_caches(&body_cache_path, &metadata_cache_path, resp)?;
                     let json = serde_json::from_slice(&bytes)?;
                     Ok(json)
+                } else if resp.status() == StatusCode::NOT_MODIFIED {
+                    from_file_cache(&body_cache_path)
                 } else {
                     log_server_error(resp);
-                    from_file_cache(&cache_path)
+                    from_file_cache(&body_cache_path)
                 }
             }
             Err(err) => {
@@ -379,7 +411,7 @@ pub extern "C" fn get_interior_ref_list_by_shop_id(
                     "get_interior_ref_list_by_shop_id api request error: {}",
                     err
                 );
-                from_file_cache(&cache_path)
+                from_file_cache(&body_cache_path)
             }
         }
     }

src/merchandise_list.rs

@@ -1,7 +1,7 @@
 use std::{ffi::CStr, ffi::CString, os::raw::c_char, slice};
 
 use anyhow::Result;
-use reqwest::Url;
+use reqwest::{StatusCode, Url};
 use serde::{Deserialize, Serialize};
 
 #[cfg(not(test))]
@@ -10,18 +10,19 @@ use log::{error, info};
 use std::{println as info, println as error};
 
 use crate::{
-    cache::file_cache_dir, cache::from_file_cache, cache::update_file_cache, log_server_error,
-    result::FFIResult,
+    cache::file_cache_dir, cache::from_file_cache, cache::load_metadata_from_file_cache,
+    cache::update_file_cache, cache::update_file_caches, cache::update_metadata_file_cache,
+    log_server_error, result::FFIResult,
 };
 
-#[derive(Serialize, Deserialize, Debug)]
+#[derive(Serialize, Deserialize, Debug, Clone)]
 pub struct MerchandiseList {
     pub id: Option<i32>,
     pub shop_id: i32,
     pub form_list: Vec<Merchandise>,
 }
 
-#[derive(Serialize, Deserialize, Debug)]
+#[derive(Serialize, Deserialize, Debug, Clone)]
 pub struct Merchandise {
     pub mod_name: String,
     pub local_form_id: u32,
@@ -77,6 +78,7 @@ pub struct RawMerchandiseVec {
     pub cap: usize,
 }
 
+// TODO: delete me if unused
 #[no_mangle]
 pub extern "C" fn create_merchandise_list(
     api_url: *const c_char,
@@ -116,13 +118,20 @@ pub extern "C" fn create_merchandise_list(
             .json(&merchandise_list)
             .send()?;
         info!("create merchandise_list response from api: {:?}", &resp);
+
+        let cache_dir = file_cache_dir(api_url)?;
+        let headers = resp.headers().clone();
         let bytes = resp.bytes()?;
         let json: MerchandiseList = serde_json::from_slice(&bytes)?;
         if let Some(id) = json.id {
             update_file_cache(
-                &file_cache_dir(api_url)?.join(format!("merchandise_list_{}.json", id)),
+                &cache_dir.join(format!("merchandise_list_{}.json", id)),
                 &bytes,
             )?;
+            update_metadata_file_cache(
+                &cache_dir.join(format!("merchandise_list_{}_metadata.json", id)),
+                &headers,
+            )?;
         }
         Ok(json)
     }
@@ -194,14 +203,13 @@ pub extern "C" fn update_merchandise_list(
             .json(&merchandise_list)
             .send()?;
         info!("update merchandise_list response from api: {:?}", &resp);
-        let bytes = resp.bytes()?;
+
+        let cache_dir = file_cache_dir(api_url)?;
+        let body_cache_path = cache_dir.join(format!("shops_{}_merchandise_list.json", shop_id));
+        let metadata_cache_path =
+            cache_dir.join(format!("shops_{}_merchandise_list_metadata.json", shop_id));
+        let bytes = update_file_caches(&body_cache_path, &metadata_cache_path, resp)?;
         let json: MerchandiseList = serde_json::from_slice(&bytes)?;
-        if let Some(id) = json.id {
-            update_file_cache(
-                &file_cache_dir(api_url)?.join(format!("shops_{}_merchandise_list.json", id)),
-                &bytes,
-            )?;
-        }
         Ok(json)
     }
 
@@ -232,6 +240,7 @@ pub extern "C" fn update_merchandise_list(
     }
 }
 
+// TODO: delete me if unused
 #[no_mangle]
 pub extern "C" fn get_merchandise_list(
     api_url: *const c_char,
@@ -256,25 +265,38 @@ pub extern "C" fn get_merchandise_list(
         info!("api_url: {:?}", url);
 
         let client = reqwest::blocking::Client::new();
-        let cache_path =
-            file_cache_dir(api_url)?.join(format!("merchandise_list_{}.json", merchandise_list_id));
+        let cache_dir = file_cache_dir(api_url)?;
+        let body_cache_path =
+            cache_dir.join(format!("merchandise_list_{}.json", merchandise_list_id));
+        let metadata_cache_path = cache_dir.join(format!(
+            "merchandise_list_{}_metadata.json",
+            merchandise_list_id
+        ));
+        let mut request = client.get(url).header("Api-Key", api_key);
+        // TODO: load metadata from in-memory LRU cache first before trying to load from file
+        if let Ok(metadata) = load_metadata_from_file_cache(&metadata_cache_path) {
+            if let Some(etag) = metadata.etag {
+                request = request.header("If-None-Match", etag);
+            }
+        }
 
-        match client.get(url).header("Api-Key", api_key).send() {
+        match request.send() {
             Ok(resp) => {
                 info!("get_merchandise_list response from api: {:?}", &resp);
                 if resp.status().is_success() {
-                    let bytes = resp.bytes()?;
-                    update_file_cache(&cache_path, &bytes)?;
+                    let bytes = update_file_caches(&body_cache_path, &metadata_cache_path, resp)?;
                     let json = serde_json::from_slice(&bytes)?;
                     Ok(json)
+                } else if resp.status() == StatusCode::NOT_MODIFIED {
+                    from_file_cache(&body_cache_path)
                 } else {
                     log_server_error(resp);
-                    from_file_cache(&cache_path)
+                    from_file_cache(&body_cache_path)
                 }
             }
             Err(err) => {
                 error!("get_merchandise_list api request error: {}", err);
-                from_file_cache(&cache_path)
+                from_file_cache(&body_cache_path)
             }
         }
     }
@@ -336,28 +358,38 @@ pub extern "C" fn get_merchandise_list_by_shop_id(
         info!("api_url: {:?}", url);
 
         let client = reqwest::blocking::Client::new();
-        let cache_path =
-            file_cache_dir(api_url)?.join(format!("shops_{}_merchandise_list.json", shop_id));
+        let cache_dir = file_cache_dir(api_url)?;
+        let body_cache_path = cache_dir.join(format!("shops_{}_merchandise_list.json", shop_id));
+        let metadata_cache_path =
+            cache_dir.join(format!("shops_{}_merchandise_list_metadata.json", shop_id));
+        let mut request = client.get(url).header("Api-Key", api_key);
+        // TODO: load metadata from in-memory LRU cache first before trying to load from file
+        if let Ok(metadata) = load_metadata_from_file_cache(&metadata_cache_path) {
+            if let Some(etag) = metadata.etag {
+                request = request.header("If-None-Match", etag);
+            }
+        }
 
-        match client.get(url).header("Api-Key", api_key).send() {
+        match request.send() {
             Ok(resp) => {
                 info!(
                     "get_merchandise_list_by_shop_id response from api: {:?}",
                     &resp
                 );
                 if resp.status().is_success() {
-                    let bytes = resp.bytes()?;
-                    update_file_cache(&cache_path, &bytes)?;
+                    let bytes = update_file_caches(&body_cache_path, &metadata_cache_path, resp)?;
                     let json = serde_json::from_slice(&bytes)?;
                     Ok(json)
+                } else if resp.status() == StatusCode::NOT_MODIFIED {
+                    from_file_cache(&body_cache_path)
                 } else {
                     log_server_error(resp);
-                    from_file_cache(&cache_path)
+                    from_file_cache(&body_cache_path)
                 }
             }
             Err(err) => {
                 error!("get_merchandise_list_by_shop_id api request error: {}", err);
-                from_file_cache(&cache_path)
+                from_file_cache(&body_cache_path)
             }
         }
     }

src/owner.rs

@@ -9,7 +9,10 @@ use log::{error, info};
 #[cfg(test)]
 use std::{println as info, println as error};
 
-use crate::{cache::file_cache_dir, cache::update_file_cache, result::FFIResult};
+use crate::{
+    cache::file_cache_dir, cache::update_file_cache, cache::update_file_caches,
+    cache::update_metadata_file_cache, result::FFIResult,
+};
 
 #[derive(Serialize, Deserialize, Debug)]
 pub struct Owner {
@@ -69,12 +72,16 @@ pub extern "C" fn create_owner(
                 .json(&owner)
                 .send()?;
             info!("create owner response from api: {:?}", &resp);
+
+            let cache_dir = file_cache_dir(api_url)?;
+            let headers = resp.headers().clone();
             let bytes = resp.bytes()?;
             let json: Owner = serde_json::from_slice(&bytes)?;
             if let Some(id) = json.id {
-                update_file_cache(
-                    &file_cache_dir(api_url)?.join(format!("owner_{}.json", id)),
-                    &bytes,
+                update_file_cache(&cache_dir.join(format!("owner_{}.json", id)), &bytes)?;
+                update_metadata_file_cache(
+                    &cache_dir.join(format!("owner_{}_metadata.json", id)),
+                    &headers,
                 )?;
             }
             Ok(json)
@@ -144,14 +151,12 @@ pub extern "C" fn update_owner(
                 .json(&owner)
                 .send()?;
             info!("update owner response from api: {:?}", &resp);
-            let bytes = resp.bytes()?;
+
+            let cache_dir = file_cache_dir(api_url)?;
+            let body_cache_path = cache_dir.join(format!("owner_{}.json", id));
+            let metadata_cache_path = cache_dir.join(format!("owner_{}_metadata.json", id));
+            let bytes = update_file_caches(&body_cache_path, &metadata_cache_path, resp)?;
             let json: Owner = serde_json::from_slice(&bytes)?;
-            if let Some(id) = json.id {
-                update_file_cache(
-                    &file_cache_dir(api_url)?.join(format!("owner_{}.json", id)),
-                    &bytes,
-                )?;
-            }
             Ok(json)
         } else {
             Err(anyhow!("api-key not defined"))

src/shop.rs

@@ -1,7 +1,7 @@
 use std::{convert::TryFrom, ffi::CStr, ffi::CString, os::raw::c_char};
 
 use anyhow::{anyhow, Result};
-use reqwest::Url;
+use reqwest::{StatusCode, Url};
 use serde::{Deserialize, Serialize};
 
 #[cfg(not(test))]
@@ -10,8 +10,9 @@ use log::{error, info};
 use std::{println as info, println as error};
 
 use crate::{
-    cache::file_cache_dir, cache::from_file_cache, cache::update_file_cache, log_server_error,
-    result::FFIResult,
+    cache::file_cache_dir, cache::from_file_cache, cache::load_metadata_from_file_cache,
+    cache::update_file_cache, cache::update_file_caches, cache::update_metadata_file_cache,
+    log_server_error, result::FFIResult,
 };
 
 #[derive(Serialize, Deserialize, Debug)]
@@ -110,12 +111,16 @@ pub extern "C" fn create_shop(
             .json(&shop)
             .send()?;
         info!("create shop response from api: {:?}", &resp);
+
+        let cache_dir = file_cache_dir(api_url)?;
+        let headers = resp.headers().clone();
         let bytes = resp.bytes()?;
         let json: Shop = serde_json::from_slice(&bytes)?;
         if let Some(id) = json.id {
-            update_file_cache(
-                &file_cache_dir(api_url)?.join(format!("shop_{}.json", id)),
-                &bytes,
+            update_file_cache(&cache_dir.join(format!("shop_{}.json", id)), &bytes)?;
+            update_metadata_file_cache(
+                &cache_dir.join(format!("shop_{}_metadata.json", id)),
+                &headers,
             )?;
         }
         Ok(json)
@@ -179,14 +184,12 @@ pub extern "C" fn update_shop(
             .json(&shop)
             .send()?;
         info!("update shop response from api: {:?}", &resp);
-        let bytes = resp.bytes()?;
+
+        let cache_dir = file_cache_dir(api_url)?;
+        let body_cache_path = cache_dir.join(format!("shop_{}.json", id));
+        let metadata_cache_path = cache_dir.join(format!("shop_{}_metadata.json", id));
+        let bytes = update_file_caches(&body_cache_path, &metadata_cache_path, resp)?;
         let json: Shop = serde_json::from_slice(&bytes)?;
-        if let Some(id) = json.id {
-            update_file_cache(
-                &file_cache_dir(api_url)?.join(format!("shop_{}.json", id)),
-                &bytes,
-            )?;
-        }
         Ok(json)
     }
 
@@ -237,24 +240,34 @@ pub extern "C" fn get_shop(
         info!("api_url: {:?}", url);
 
         let client = reqwest::blocking::Client::new();
-        let cache_path = file_cache_dir(api_url)?.join(format!("shop_{}.json", shop_id));
+        let cache_dir = file_cache_dir(api_url)?;
+        let body_cache_path = cache_dir.join(format!("shop_{}.json", shop_id));
+        let metadata_cache_path = cache_dir.join(format!("shop_{}_metadata.json", shop_id));
+        let mut request = client.get(url).header("Api-Key", api_key);
+        // TODO: load metadata from in-memory LRU cache first before trying to load from file
+        if let Ok(metadata) = load_metadata_from_file_cache(&metadata_cache_path) {
+            if let Some(etag) = metadata.etag {
+                request = request.header("If-None-Match", etag);
+            }
+        }
 
-        match client.get(url).header("Api-Key", api_key).send() {
+        match request.send() {
             Ok(resp) => {
                 info!("get_shop response from api: {:?}", &resp);
                 if resp.status().is_success() {
-                    let bytes = resp.bytes()?;
-                    update_file_cache(&cache_path, &bytes)?;
+                    let bytes = update_file_caches(&body_cache_path, &metadata_cache_path, resp)?;
                     let json = serde_json::from_slice(&bytes)?;
                     Ok(json)
+                } else if resp.status() == StatusCode::NOT_MODIFIED {
+                    from_file_cache(&body_cache_path)
                 } else {
                     log_server_error(resp);
-                    from_file_cache(&cache_path)
+                    from_file_cache(&body_cache_path)
                 }
             }
             Err(err) => {
                 error!("get_shop api request error: {}", err);
-                from_file_cache(&cache_path)
+                from_file_cache(&body_cache_path)
             }
         }
     }
@@ -301,24 +314,34 @@ pub extern "C" fn list_shops(
         info!("api_url: {:?}", url);
 
         let client = reqwest::blocking::Client::new();
-        let cache_path = file_cache_dir(api_url)?.join("shops.json");
+        let cache_dir = file_cache_dir(api_url)?;
+        let body_cache_path = cache_dir.join("shops.json");
+        let metadata_cache_path = cache_dir.join("shops_metadata.json");
+        let mut request = client.get(url).header("Api-Key", api_key);
+        // TODO: load metadata from in-memory LRU cache first before trying to load from file
+        if let Ok(metadata) = load_metadata_from_file_cache(&metadata_cache_path) {
+            if let Some(etag) = metadata.etag {
+                request = request.header("If-None-Match", etag);
+            }
+        }
 
-        match client.get(url).header("Api-Key", api_key).send() {
+        match request.send() {
             Ok(resp) => {
                 info!("list_shops response from api: {:?}", &resp);
                 if resp.status().is_success() {
-                    let bytes = resp.bytes()?;
-                    update_file_cache(&cache_path, &bytes)?;
+                    let bytes = update_file_caches(&body_cache_path, &metadata_cache_path, resp)?;
                     let json = serde_json::from_slice(&bytes)?;
                     Ok(json)
+                } else if resp.status() == StatusCode::NOT_MODIFIED {
+                    from_file_cache(&body_cache_path)
                 } else {
                     log_server_error(resp);
-                    from_file_cache(&cache_path)
+                    from_file_cache(&body_cache_path)
                 }
             }
             Err(err) => {
                 error!("list_shops api request error: {}", err);
-                from_file_cache(&cache_path)
+                from_file_cache(&body_cache_path)
             }
         }
     }

src/transaction.rs

@@ -11,8 +11,8 @@ use log::{error, info};
 use std::{println as info, println as error};
 
 use crate::{
-    cache::file_cache_dir, cache::from_file_cache, cache::update_file_cache, log_server_error,
-    result::FFIResult,
+    cache::file_cache_dir, cache::from_file_cache, cache::update_file_cache,
+    cache::update_metadata_file_cache, log_server_error, result::FFIResult,
 };
 
 #[derive(Serialize, Deserialize, Debug)]
@@ -157,31 +157,43 @@ pub extern "C" fn create_transaction(
             .json(&transaction)
             .send()?;
         info!("create transaction response from api: {:?}", &resp);
+
+        let cache_dir = file_cache_dir(api_url)?;
+        let headers = resp.headers().clone();
         let status = resp.status();
         let bytes = resp.bytes()?;
         if status.is_success() {
             let json: Transaction = serde_json::from_slice(&bytes)?;
             if let Some(id) = json.id {
-                update_file_cache(
-                    &file_cache_dir(api_url)?.join(format!("transaction_{}.json", id)),
-                    &bytes,
+                update_file_cache(&cache_dir.join(format!("transaction_{}.json", id)), &bytes)?;
+                update_metadata_file_cache(
+                    &cache_dir.join(format!("transaction_{}_metadata.json", id)),
+                    &headers,
                 )?;
             }
             Ok(json)
         } else {
+            // TODO: abstract this away into a separate helper
             match serde_json::from_slice::<HttpApiProblem>(&bytes) {
                 Ok(api_problem) => {
                     let detail = api_problem.detail.unwrap_or("".to_string());
-                    error!("Server {} error: {}. {}", status, api_problem.title, detail);
+                    error!(
+                        "Server {}: {}. {}",
+                        status.as_u16(),
+                        api_problem.title,
+                        detail
+                    );
                     Err(anyhow!(format!(
-                        "Server {} error: {}. {}",
-                        status, api_problem.title, detail
+                        "Server {}: {}. {}",
+                        status.as_u16(),
+                        api_problem.title,
+                        detail
                     )))
                 }
                 Err(_) => {
                     let detail = str::from_utf8(&bytes).unwrap_or("unknown");
-                    error!("Server {} error: {}", status, detail);
-                    Err(anyhow!(format!("Server {} error: {}", status, detail)))
+                    error!("Server {}: {}", status.as_u16(), detail);
+                    Err(anyhow!(format!("Server {}: {}", status.as_u16(), detail)))
                 }
             }
         }
@@ -293,7 +305,15 @@ mod tests {
     fn test_create_transaction_server_error() {
         let mock = mock("POST", "/v1/transactions")
             .with_status(500)
-            .with_body("Internal Server Error")
+            .with_header("content-type", "application/problem+json")
+            .with_body(
+                r#"{
+                "detail": "Some error detail",
+                "instance": "https://httpstatuses.com/500",
+                "status": 500,
+                "title": "Internal Server Error"
+            }"#,
+            )
             .create();
 
         let api_url = CString::new("url").unwrap().into_raw();
@@ -323,7 +343,7 @@ mod tests {
             FFIResult::Err(error) => {
                 assert_eq!(
                     unsafe { CStr::from_ptr(error).to_string_lossy() },
-                    "expected value at line 1 column 1"
+                    "Server 500: Internal Server Error. Some error detail"
                 );
             }
         }