Hi Thomas,
this should only happen when such queries dynamically access collections,
so that it is not clear at query start with collections will be accessed.
This may be the case for graph traversals or for queries that invoke the
`DOCUMENT()` AQL function using runtime expressions or variables.
Consider the following example:
Query 1 (the read-only query) accesses collection A (which will be
read-locked when the query starts)
Query 2 (some other write query) may now start and try to write-lock
collections B and A.
Query 2 may succeed in getting the lock for B, but not for A (because query
1 has got it).
If Query 1 now dynamically needs to access collection B, then none of the
two queries would make any progress:
- query 1 would need to wait for the lock on collection B (which is held by
query 2),
- query 2 would need to wait for the lock on collection A (which is held by
query 1)
This is when the deadlock detection starts and will abort one of the
queries. This can also be a read-only query.
Deadlocks should not happen if all collections needed by a query can be
determined at query compile time. Then the locking order should be fully
deterministic and deadlock-free.
But when collections are accessed dynamically, there can be no
deterministic locking order.
A workaround for avoiding deadlocks is to specify extra collections (those
that will be accessed dynamically) in a `WITH` clause at the beginning of
an AQL query, e.g.
WITH extracollection1, extracollection2
FOR doc IN collection
...
Best regards
Jan
Am Mittwoch, 5. April 2017 04:59:53 UTC+2 schrieb Thomas Weiss:
>
> Hi there,
>
> I've just seen in my logs that requests have failed with the "deadlock
> detected (while executing)" message, but those requests were all read-only
> AQL queries. How can such requests be involved in a deadlock?
>
> Thanks,
> Thomas
>
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