Error handling

WiredTiger operations return a value of 0 on success and a non-zero value on error. Error codes may be either positive or negative: positive error codes are standard error codes as described for POSIX-like systems (for example, EINVAL or EBUSY), negative error codes are WiredTiger-specific (for example, WT_ROLLBACK).

EBUSY errors

WiredTiger returns EBUSY for operations requiring exclusive access, when an object is not available for exclusive access. These operations include the WT_SESSION::alter, WT_SESSION::drop, WT_SESSION::rename, WT_SESSION::salvage, WT_SESSION::upgrade and WT_SESSION::verify methods, all of which will return EBUSY and fail if there are open cursors on the target object. Internal WiredTiger threads may temporarily open cursors on objects (for example, threads performing operations like statistics logging), and in that case operations may temporarily fail and return EBUSY when there are no application cursors open on the object. In this case, simply retrying the operation should be sufficient.

Additionally, unwritten data in the WiredTiger cache will prevent exclusive access to objects. In this case, calling the WT_SESSION:checkpoint method to perform a database checkpoint should resolve the problem, allowing a subsequent retry of the operation requiring exclusive access to succeed. Further failures imply other threads of control simultaneously updating the object in cache. Repeatedly calling checkpoint will race with those threads, and it's unspecified when or even if exclusive access to the object will be granted. Generally, applications will not call WiredTiger methods requiring exclusive access when the objects might be in active use by other threads.

WiredTiger-specific errors

WiredTiger-specific error codes are allocated from -31,800 to -31,999, inclusive. The following is a list of the WiredTiger-specific return values:

WT_ROLLBACK

This error is generated when an operation cannot be completed due to a conflict with concurrent operations. The operation may be retried; if a transaction is in progress, it should be rolled back and the operation retried in a new transaction.

WT_DUPLICATE_KEY

This error is generated when the application attempts to insert a record with the same key as an existing record without the 'overwrite' configuration to WT_SESSION::open_cursor.

WT_ERROR

This error is returned when an error is not covered by a specific error return. The operation may be retried; if a transaction is in progress, it should be rolled back and the operation retried in a new transaction.

WT_NOTFOUND

This error indicates an operation did not find a value to return. This includes cursor search and other operations where no record matched the cursor's search key such as WT_CURSOR::update or WT_CURSOR::remove.

WT_PANIC

This error indicates an underlying problem that requires a database restart. The application may exit immediately, no further WiredTiger calls are required (and further calls will themselves immediately fail).

WT_RUN_RECOVERY

This error is generated when wiredtiger_open is configured to return an error if recovery is required to use the database.

WT_CACHE_FULL

This error is only generated when wiredtiger_open is configured to run in-memory, and an insert or update operation requires more than the configured cache size to complete, or when an application thread fails to do eviction within cache_max_wait_ms. The operation may be retried; if a transaction is in progress, it should be rolled back and the operation retried in a new transaction.

WT_PREPARE_CONFLICT

This error is generated when the application attempts to read an updated record which is part of a transaction that has been prepared but not yet resolved.

WT_TRY_SALVAGE

This error is generated when corruption is detected in an on-disk file. During normal operations, this may occur in rare circumstances as a result of a system crash. The application may choose to salvage the file or retry wiredtiger_open with the 'salvage=true' configuration setting.

Translating errors

The WT_SESSION::strerror and wiredtiger_strerror functions return the standard text message associated with any WiredTiger, ISO C, or POSIX standard API.

        const char *key = "non-existent key";
        cursor->set_key(cursor, key);
        if ((ret = cursor->remove(cursor)) != 0)
            fprintf(stderr, "cursor.remove: %s\n", cursor->session->strerror(cursor->session, ret));

        const char *key = "non-existent key";
        cursor->set_key(cursor, key);
        if ((ret = cursor->remove(cursor)) != 0)
            fprintf(stderr, "cursor.remove: %s\n", wiredtiger_strerror(ret));

Note that wiredtiger_strerror is not thread-safe.