The basic example

If you want to execute the three components a, b, c in parallel, you can use the WHEN keyword. It should be noted that WHEN must be capitalized.

<chain name="chain1">
    WHEN(a, b, c);
</chain>

Nested with serial (1)

Let's combine THEN and WHEN to see an example:

<chain name="chain1">
    THEN(
        a,
        WHEN(b, c, d),
        e
    );
</chain>

Flow chart

In the above example, e will be executed only after b, c, and d.

Nested with serial (2)

The above example should be well understood, so let's look at another example:

<chain name="chain1">
    THEN(
        a,
        WHEN(b, THEN(c, d)),
        e
    );
</chain>

Flow chart

Ignore errors

The WHEN keyword provides a subkeyword ignoreError (defaults to false) to provide the feature of ignoring errors. The usage is as follows:

<chain name="chain1">
    THEN(
        a,
        WHEN(b, c, d).ignoreError(true),
        e
    );
</chain>

Flow chart

The above assumes that any node in b, c, and d has an exception, then eventually e will still be executed.

Any node is executed first and other nodes are ignored

The WHEN keyword provides a sub-keyword any (default is false) to provide the feature that in the parallel process, any node is executed first, ignoring other node and continuing to execute. The usage is as follows:

<chain name="chain1">
    THEN(
        a,
        WHEN(b, THEN(c, d), e).any(true),
        f
    );
</chain>

Flow chart

In the above process, if node e is executed first, then node f will be executed immediately regardless of whether other nodes are executed.

The concept of groups

In previous versions, parallel mode had the concept of groups, and in version 2.8.X, we removed the concept of groups.

With EL expressions, you actually write 2 different WHENs as 2 groups. for example:

<chain name="chain1">
    THEN(
        WHEN(a, b, c, d)
    );
</chain>

The above abcd are all in the same parallel group.

<chain name="chain1">
    THEN(
        WHEN(a, b)，
        WHEN(c, d)
    );
</chain>

In the above example, ab is a parallel group and cd is another parallel group.