How to pass a single DataLoader instance to all graphql request handlers?

[ryancole]

I'm trying to implement DataLoader which batches together underlying database queries within a single request. I'm looking towards react-relay-network-layer to assist with the batching of GraphQL queries into a single HTTP request so that I can use a single DataLoader for all queries within that request.

I notice that graphqlBatchHTTPWrapper calls the graphqlHTTPMiddleware for each query. I need to make accessible to each graphqlHTTPMiddleware call the same instance of my loaders. How do you suggest that I go about this?

Prior to adding react-relay-network-layer, or any batching, just using the default relay network layer, I was simply instantiating my DataLoaders right within the express-graphql middleware function and passing them to the resolvers via rootValue. I can no longer do that there because that'll create new DataLoaders for each query. I need a single DataLoader per request.

Looking for your advice. Thanks!

[nodkz]

Please provide your prior realization with express-graphql. It helps me point you to right existed solution, or create new one based on your example.

[ryancole]

Sorry about how possibly verbose this explanation is, but I'm just trying to provide enough to show you my situation.

Here is a link to where I use the react-relay-network-layer GraphQL-related code, so that I can support batched queries (in a single HTTP request) on the GraphQL side of things ...

https://github.com/ryancole/league/blob/dataloader/src/graphql/server.js#L22-L30

And here is the express-graphql middleware handler in which you would normally handle a single HTTP request to the default graphql-express handler, but in my code I've wrapped using react-relay-network-layer ...

https://github.com/ryancole/league/blob/dataloader/src/graphql/util/handleGraphqlRequest.js#L7

That code above shows where I create my DataLoader instances.

Now, my intention is for these DataLoader instances to be made available to express-graphql via rootValue but in such a way that one instance of them is used per request. The code that I linked does not work, because the express-graphql middleware function is executed per query, as opposed to per request, when using react-relay-network-layer.

[ryancole]

Just another reply trying to explain my issue.

I'm needing to be able to provide rootValue but a single instance of it to all express-graphql handlers. I essentially need to create a variable, pass it through react-relay-network-layer, and into each request handler.

[nodkz]

Awesome explanation, straight what I wanted!

Right now does not exists solution for your case. But it's not hard for implementation.
I now prepare proposal for review.

[ryancole]

I think I could just make an express middleware that pins my DataLoaders to the request object, before the react-relay-network-layer code runs, which would make them available to my GraphQL middleware down the line. That wouldn't require any changes to RRNL.

Edit: Although I'm not sure how logical that is, considering the loaders have nothing to do with the request itself and probably don't need to logically be associated with the request object.

[ryancole]

Alright well, I went ahead and made this express middleware that creates my DataLoader instances, once per request, and pins them to the request object.

https://github.com/ryancole/league/blob/dataloader/src/graphql/util/expressDataLoaderMiddleware.js

I'm not sure that it is technically logical, but it gets me moving forward. If you think that you've got a better idea about how to possibly pass parameters through react-relay-network-layer's express middleware (possibly HOF) then I'll just keep checking back! Thanks!

[nodkz]

Finally, I found the same solution as you provide, based on your first detailed explanation.

I just comitted example: https://github.com/nodkz/react-relay-network-layer/blob/master/examples/dataLoaderPerBatchRequest.js

Please review it and leave a message, if found some errors.
And many thanks for your issue, help and examples. I'll use same code in my project.

If no any additions, you may close issue.

[nodkz]

Will be great if you fix typos and misunderstanding places in comments, right via github web-editor https://github.com/nodkz/react-relay-network-layer/edit/master/examples/dataLoaderPerBatchRequest.js

[nodkz]

And last question, why do you pass DataLoader to rootValue instead of context?
I think that context is the better place for such per request things by graphql design.

[ryancole]

@nodkz Regarding your question about rootValue versus context. Based on my understanding, context is for session based data, such as current user, etc, while rootValue is for "everything else." The spec leaves this pretty open ended, and I made my decision based on some github comments by graphql contributors. I don't have links handy, but that's my recollection. I consider these data fetching functions to not be session related and more categorized as "everything else." :)

I'll go over your adjustments in just a few minutes.

[taion]

BTW, compare my PR adding this functionality to express-graphql.

[nodkz]

@ryancole thanks for answer! Really this is a tricky question. I'm confusing, but till not change my decision due context word meaning 😜 And all that based on user request, should be in context, even fetchers.

Asked Lee for advice with context and rootValue https://twitter.com/nodkz/status/756378662711267328

[ryancole]

@nodkz true. I personally don't see the clear difference between context and rootValue. That was a somewhat ambiguous change to GraphQL, imo.

@taion I'll check it out, thanks. @nodkz, I do believe this is the PR that taion is talking about: graphql/express-graphql#100

[nodkz]

DataLoaders should be in context

[taion]

IMO you should really share both rootValue and context across all queries in a batch... I guess depending on the semantics you want. Like really a "batched query" is just a single query that happens to have multiple query roots, with some workarounds for various Relay compat reasons.

[nodkz]

There are no restrictions in sharing: https://github.com/nodkz/react-relay-network-layer/edit/master/examples/dataLoaderPerBatchRequest.js - one config for all queries per one http request.

We just try choose better place for DL.

[taion]

No, I mean that there's not really much reason to not always share context and rootValue across all GraphQL requests in a batched HTTP request.

[nodkz]

Ah, completely agreed with you.

Too scary and complex config for readme.md. I wanted write something hacky again. But I'll prefer wait your PR: graphql/express-graphql#100 And remove tons of my rubbish code.

Will pray 😉 that @leebyron accept it.