Re: Enforcing field type on publish
Alpert, Reed <reed.alpert@...>
I agree, the callback to a module that can do publish checking/etc is good.
I prototyped a checking module, and it works, although loading the dictionary on 1st publish has a small delay, and also required that the publish not be from a mama timer callback, since that deadlocked either on sending the subscription on the throttle queue, or getting the dict back.
We actually have more requirements for publish that are very specific to our env (adding system audit fields to all publishes to track when/where the data comes from).
Having a callback we can engage would allow us to do this w/o any other users needing to have our stuff in the way.
So loading our module at runtime and calling it would work quite well.
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From: Glenn McClements [mailto:gmcclements@...]
I like #2 more, but I still want to keep the more data model/market data/custom logic away from the core messaging part of OpenMAMA.
What I’m thinking of is a hook/callback to a programmer defined function that would take the publisher, transport, message, a user defined closure etc that would return a status to indicate if the message should be published or not. The dictionary etc could be obtained at initialisation time through a separate init function.
This dovetails with some of the other pieces of work going on:
I still need to work though this a bit more but let me know what you think.
<Alpert>, Reed <reed.alpert@...>
Yes I agree with Ben’s comments.
Here are some caveats:
1. This is a feature that is requested by our market data infrastructure team to prevent app dev teams from accidentally publishing incorrect field types. We don’t believe that a voluntary program will work, which is why the FieldCache as a location is not preferred. Apps won’t use the FieldCache if they don’t need it, or are using an Excel plugin for publishing (quite common).
2. This is a configured feature, and only for publication.
3. This is not for subscription, that is, a subscriber can use any of the getXX() methods that work.
There seems to be general agreement that the OM layer is much better place for this feature than bridges themselves.
The two best locations seem to be:
1. In mamaMsg, in the addXX and updateXX methods, probably with a macro like CHECK_MODIFY() that calls out to another module to do the work.
a. This is good since it keep the message construction in mamaMsg module.
b. This is good since it give the programmer the quickest feedback on an error condition.
c. This is bad since it puts some performance hit on construction of every message, even on subscribe.
d. This is bad since it makes the code active for creating subscribe messages too.
e. This is bad since it puts more code in more places for this feature.
2. In publisher.send() that checks a config var and then calls out to another module to do the work.
a. This is good since it has minimal code changes, and minimal performance effect.
b. This is good since it only affects publish messages.
c. This is bad since it gives delayed failure, and there may be multiple fields that fail.
i. Probably logging each failed field and returning an error code will work well to help developers fix problems.
I think the publisher.send() method is the best place, given the above.
But figuring out which dictionary to use for the checking seems to be the most difficult part.
Adding this feature as a config option may require also adding some optional config info about how to get a dictionary for a specific bridge, e.g., the source name.
Once the dictionary loading/mapping is solved then the rest seems fairly straightforward, except for the determination of allowed/denied conversions as configurable policy, but starting with 3 simple ones seems best:
1. No checking (default)
2. Type must match dictionary.
3. Type must match dictionary, with a small number of allowed conversions that do not incur data loss (e.g., F32 -> F64).
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