Read-only thread safety of existing implementations

[Sebastiano Vigna]

I have seen that there is a specific wrapper to access concurrently a Graph with read/write operations. However, I could find no information about the read-only thread safety of classes such as DefaultDirectedGraph or FastutilMapGraph. I think it would be useful to know from the class documentation whether they are thread safe for read-only operations, or not. The succinct implementations I'm providing are thread-safe, whereas the adapters for WebGraph graphs provide a lightweight copy mechanism

Do you think it is useful to give this kind of information to the user?

Ciao,

seba

[John Sichi]

For the default graph implementations, we resolved this one in:

https://github.com/jgrapht/jgrapht/issues/520

so at this point we can declare them as safe for concurrent reads in the absence of concurrent writes.  I'll let Dimitrios answer regarding the Fastutil-based implementations.

We don't want to make any guarantees on the Graph interface itself, so we should add a note on every implementation class (plus the builders), as well as in the user guide.

[Sebastiano Vigna]

I agree—I definitely would not add something like that to an interface (my adapters wouldn't comply!). But if two years ago you made it thread safe it is important that this is documented.

We are indeed using JGraphT in a EU project and we were wondering about thread safety. Knowing about the fastutil-based implementations would be even more useful. One would also expect the sparse implementations, being immutable, to be thread safe.

seba

[Dimitrios Michail]

I would make a wild guess and claim that the fastutil versions also behave in a similar way as our default implementation. 

They mostly just swap the underlying hash table implementations. They do not introduce any more laziness.

Our sparse graphs are also thread-safe for read-only operations.

No idea about the guava adapters.

We should probably document this in the graph implementations.

Best, 

Dimitrios

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[John Sichi]

I've created a PR here.  https://github.com/jgrapht/jgrapht/pull/1005