Handling Errors
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Errors are inevitable. Rust offers several flexible approaches to handling them.
The developer’s job is to be prepared for whatever is likely to come up — and to try and be prepared for anything that conceivably could come up. Couchbase gives you a lot of flexibility, but it is recommended that you equip yourself with an understanding of the possibilities.
How errors are actually returned (e.g. via Try, Future, or Mono) and handled are covered here, so this document will focus instead on specific errors, along with a broader look at error handling strategies.
Error Model
The Rust SDK tries to make handling errors as easy as possible, whilst also attaching as much useful information as possible to the error to aid in debugging.
Errors are a wrapper around a accessible ErrorKind enum, and an opaque ErrorContext.
The ErrorKind is large enum containing a wide variety of error types, but most likely you will only want to handle a subset of them based on your requirements.
Key-Value Errors
The KV Service exposes several common errors that can be encountered - both during development, and to be handled by the production app. Here we will cover some of the most common errors.
If a particular key cannot be found it is returned as a DocumentNotFound:
let doc = json!({
"foo": "bar",
"baz": "qux",
});
match collection.replace("does-not-exist", doc, None).await {
Ok(_result) => {
println!("Document upsert successful");
}
Err(e) => match e.kind() {
couchbase::error::ErrorKind::DocumentNotFound => {
println!("Document not found");
}
_ => println!("Error: {e}"),
},
}match collection.get("does-not-exist", None).await {
Ok(_result) => {
println!("Document get successful");
}
Err(e) => match e.kind() {
couchbase::error::ErrorKind::DocumentNotFound => {
println!("Document not found");
}
_ => println!("Error: {e}"),
},
}On the other hand if the key already exists and should not (e.g. on an insert) then it is raised as a DocumentExists:
let doc = json!({
"foo": "bar",
"baz": "qux",
});
match collection.insert("does-already-exist", doc, None).await {
Ok(_result) => {
println!("Document upsert successful");
}
Err(e) => match e.kind() {
couchbase::error::ErrorKind::DocumentExists => {
println!("Document exists");
}
_ => println!("Error: {e}"),
},
}Concurrency
Couchbase provides optimistic concurrency using CAS (Compare and Swap).
Each document gets a CAS value on the server, which is changed on each mutation.
When you get a document you automatically receive its CAS value.
When replacing the document, if you provide that CAS the server can check that the document has not been concurrently modified by another agent in-between.
If it has, it returns CasMismatch, and the most appropriate response is to simply retry it:
let mut guard = 3;
let new_json = json!({"foo": "bar"});
let doc = collection.get("doc", None).await?;
match collection
.replace("doc", &new_json, ReplaceOptions::new().cas(doc.cas()))
.await
{
Ok(_result) => {
println!("Document replace successful");
return Ok(());
}
Err(e) => match e.kind() {
couchbase::error::ErrorKind::CasMismatch => {
guard -= 1;
if guard > 0 {
println!("CAS mismatch, retry up to {guard} times");
} else {
return Err(ExamplesError::from(e));
}
}
_ => {
println!("Error: {e}");
return Err(ExamplesError::from(e));
}
},
};Ambiguity
There are situations with any distributed system in which it is simply impossible to know for sure if the operation completed successfully or not. Take this as an example: your application requests that a new document be created on Couchbase Server. This completes, but, just before the server can notify the client that it was successful, a network switch dies and the application’s connection to the server is lost. The application can drop the operation future, but it’s ambiguous to the app whether the operation succeeded or not.
Another ambiguous error is DurabilityAmbiguous, which can returned when performing a durable operation.
This similarly indicates that the operation may or may not have succeeded — though when using durability you are guaranteed that the operation will either have been applied to all replicas, or none.
Given the inevitability of ambiguity, how is the application supposed to handle this?
It really needs to be considered case-by-case, but the general strategy is to become certain if the operation succeeded or not, and to retry it if required.
For instance, for inserts, they can simply be retried to see if they fail on DocumentExists, in which case the operation was successful:
let mut guard = 3;
let doc = json!({"foo": "bar"});
loop {
let result = collection
.insert(
"doc-id",
&doc,
InsertOptions::new().durability_level(DurabilityLevel::MAJORITY),
)
.await;
match result {
Ok(_result) => {
println!("Document insert successful");
return Ok(());
}
Err(e) => match e.kind() {
couchbase::error::ErrorKind::DurabilityAmbiguous => {
// For ambiguous errors on inserts, simply retry them.
guard -= 1;
if guard > 0 {
println!("Durability ambiguous, retry up to {guard} times");
} else {
return Err(ExamplesError::from(e));
}
}
couchbase::error::ErrorKind::DocumentExists => {
// The logic here is that if we failed to insert on the first attempt then
// it's a true error, otherwise we retried due to an ambiguous error, and
// it's ok to continue as the operation was actually successful.
println!("Document insert successful");
return Ok(());
}
_ => {
println!("Error: {e}");
return Err(ExamplesError::from(e));
}
},
}
}That example is much closer to what an application will want to be doing. Let’s flesh it out further.
Real-World Error Handling
The application can write wrappers so that it can easily do operations without having to duplicate the error handling each time. Something like this:
let initial_guard = 3;
let base_delay = Duration::from_millis(10);
let mut current_backoff_factor = 1;
let mut guard = initial_guard;
let doc = json!({
"foo": "bar",
"baz": "qux",
});
let doc_id = "document-key";
loop {
let result = timeout(
Duration::from_millis(2500),
collection.insert(
&doc_id,
&doc,
InsertOptions::new().durability_level(DurabilityLevel::MAJORITY),
),
)
.await;
match result {
Ok(_result) => {
println!("Document insert successful");
return Ok(());
}
Err(e) => match e.kind() {
couchbase::error::ErrorKind::DocumentExists => {
// The logic here is that if we failed to insert on the first attempt then
// it's a true error, otherwise we retried due to an ambiguous error, and
// it's ok to continue as the operation was actually successful.
if guard == initial_guard {
return Err(ExamplesError::from(e));
}
println!("Document insert successful");
return Ok(());
}
couchbase::error::ErrorKind::DurabilityAmbiguous
// Temporary/transient errors that are likely to be resolved
// on a retry
| couchbase::error::ErrorKind::TemporaryFailure
| couchbase::error::ErrorKind::DurabilityWriteInProgress
| couchbase::error::ErrorKind::DurableWriteRecommitInProgress
// These transient errors won't be returned on an insert, but can be used
// when writing similar wrappers for other mutation operations
| couchbase::error::ErrorKind::CasMismatch => {
if guard > 0 {
tokio::time::sleep(base_delay * current_backoff_factor).await;
current_backoff_factor *= 2;
guard -= 1;
println!("Transient error, retry up to {guard} times");
} else {
return Err(ExamplesError::from(e));
}
}
_ => {
println!("Error: {e}");
return Err(ExamplesError::from(e));
}
},
}
}This will make a 'best effort' to do the insert (though its retry strategy is rather naïve, and applications may want to implement a more sophisticated approach involving exponential backoff and circuit breaking.)
If that best effort fails, and the do_insert call still returns a Error, then it’s highly context-dependent how to handle that.
Examples would include displaying a "please try again later" error to a user, if there is one, and logging it for manual human review.
The application must make a suitable call for each case.
The application can write similar wrappers for the other operations — replace, upsert, et al.
Note that the logic is a little different in each case:
for inserts, we confirm if the operation has already been successful on an ambiguous result by checking for DocumentExists
But this wouldn’t make sense for an upsert.
Idempotent and Non-Idempotent Operations
| Idempotent operations are those that can be applied multiple times and only have one effect. Repeatedly setting an email field is idempotent — increasing a counter by one is not. |
Some operations we can view as idempotent as they will fail with no effect after the first success — such as inserts.
Idempotent operations are much easier to handle, as on ambiguous error results (DurabilityAmbiguous or timeouts) the operation can simply be retried.
Most key-value operations are idempotent.
For those that aren’t, such as a Sub-Document array_append call, or a counter increment, the application should, on an ambiguous result, first read the document to see if that change was applied.
Query and Analytics Errors
A SQL++ (formerly N1QL) query either returns results or an error as the same way as KV, like so:
let statement = "SELECT * from `airport` LIMIT 10;";
match scope.query(statement, None).await {
Ok(res) => {
println!("Success!");
}
Err(e) => {
println!("Query failed: {e}");
}
};Additional Resources
Errors & Exception handling is an expansive topic. Here, we have covered examples of the kinds of exception scenarios that you are most likely to face. More fundamentally, you also need to weigh up concepts of durability.
Diagnostic methods are available to check on the health of the cluster, and the health of the network.
Logging methods are dependent upon the platform and SDK used. We offer recommendations and practical examples.
We have a listing of error messages, with some pointers to what to do when you encounter them.