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I'm working on a NodeJS project and using pretty common AWS setup it seems. My ApiGateway receives call, triggers lambda A, then this lambda A triggers other lambdas, say B or C depending on params passed from ApiGateway.

Lambda A needs to access MongoDB and to avoid hassle with running MongoDB myself I decided to use mLab. ATM Lambda A is accessing MongoDB using NodeJS driver. Now, not to start connection with every Lambda A execution I use connection pool, again, inside of Lambda A code, outside of handler I keep connection pool that allows me to reuse connections when Lambda A is invoked multiple times. This seems to work fine. However, I'm not sure how to deal with connections when Lambda A is invoking Lambda B and Lambda B needs to access mLab's MongoDB database.

  1. Is it possible to pass connection pool somehow or Lambda B would have to keep its own connection pool?
  2. I was thinking of using mLab's Data API that exposes most of the operations of MongoDB driver and so I could use HTTP calls e.g. GET and POST to run commands against database. It seems similar to RESTHeart it seems.

I'm leaning towards option 2 but on mLab's Data API it clearly states to avoid using REST api unless cannot connect using MongoDB driver directly:

The first method—the one we strongly recommend whenever possible for added performance and functionality—is to connect using one of the available MongoDB drivers. You do not need to use our API if you use the driver. The second method, documented in this article, is to connect via mLab’s RESTful Data API. Use this method only if you cannot connect using a MongoDB driver.

Given all this how would it be best to approach it? 1 or 2 or is there any other option I should consider?

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Unfortunately you won't be able to 'share' a mongo connection across lambdas because ultimately there's a 'physical' socket to the connection which is specific to that instance.

I think both of your solutions are good depending on usage.

If you tend to have steady average concurrency on both lambda A and B across an hour period (which is a bit of a rule of thumb as to how long AWS keeps a lambda instance alive), then having them both own their own static connections is a good solution. This is because the chances are that a request will reach an already started and connected lambda. I would also guess that node drivers for 'vanilla' mongo are more mature than those for the RESTFul Data API.

However if you get spikey or uneven load, then you might use the RESTFul Data API. This is because you'll be centralising the responsibility for managing the number of open connections to your instances to a single point, which under these conditions means you're less likely to be opening unneeded connections, or using all of your current capacity and having to wait for a new connection to be established.

Ultimately it's a game of probabilistic load balancing- either you 'pool' all your connections in a central place (the Data API) and become less affected by the usage of a single function at the expense of greater latency on individual operations, or you pool at a function level but are more exposed to cold-starts opening connections under uneven concurrency.

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