koalambda
v0.1.18
Published
Koa-like middleware framework to build AWS Lambda functions
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Koala-mbda or Koa-Lambda
Koa-style middleware concept to quickly build AWS Lambda functions
Middlewares
Use Koa style middlewares and the kompose
method. Examples can be found in handlers e.g. user/addToHistory.ts
A handler is therefore simply equal to the outcome of kompose
e.g.
export const handler = kompose(
callbackBased,
...,
async (ctx, next) => {
... // <- this is called while going down the middleware chain
await next()
... // <- this is called once the bottom of the chain has been reached and we are going back up the chain
},
...
)
Rules
A middleware follows the following rules:
- arguments are
ctx:EventContext
andnext
(optional, see Utility middlewares below) ctx
contains three properties:event:AWSEvent
,context:AWSContext
andstate
. The event type varies depending on the trigger of the lambda function.- All middlewares should be
async
functions; they should always callawait next()
(ornext && await next()
in the case of utility) - Code reached before the
await next()
call is processed down the middleware chain - Code reached after the
await next()
call is processed up the middleware chain - In case of errors, the middleware should
throw
an error, not return a rejected promise
Convention
- All properties to be added or modified along the middleware chain should be added/modified on the
ctx.state
object - For HTTP based calls, the top chain will expect a
ctx.state.response
property to be populated - Errors thrown follow the pattern:
{message: '...', code: 42}
Utility middlewares
In case of utility middlewares, that are expected to be used both as normal functions and as part of a middleware chain:
- the
next
parameter should be made optional (next?
) - the call to
next
should be made optional with the statementnext && await next()
E.g. withUserId
can be used as a part of a chain, when user isn't needed (only id) but it is also used within withUser
as normal function
Base middlewares
↓ means acts down the chain; ↑ up the chain
callbackBased & contextBased ↑
Decide how the outcome of the Lambda function is triggered. Http calls use callback, whereas Cognito triggers use context for example. As of this writing, one of the two must be used as the top middleware for any handler function
httpResponse & standardHttpResponse ↑
Handles Http responses, both success and error. standardHttpResponse
simply uses a default code 200 for success, as well as default CORS enabled and JSON content.
withUserId ↓
Reads the user Id from the request and adds it as ctx.state.userId
withUser ↓
Reads the user Id and loads the corresponding user object. Adds the user as ctx.state.user
Note: Does not require withUserId
to be added to the chain as it is already part of the code of withUser.
jsonBody ↓
JSON-Parses the body of a POST. Sets the parsed object back onto ctx.state.body
(overwrites original)
putInState ↓
Takes value with specified name and puts it in the state
filter ↓
filter the specified argument by specified function
Logical switches
when
Takes a "check" callback which receives the context, and a middleware. If check passes (either as a boolean or a Promise which resolves with a boolean), middleware is called.
Note: Check promise rejection is not currently considered a fail of the check
whenAttributeExists
Similar to when
but checks whether a given attribute exists on the state. Uses lodash.get
for shortcut (dot notation)
whenAttributeEquals
Similar to whenAttributeExists
but checks whether the given attribute equals the given value.
Changelog
v0.0.2:
- move all properties to state
- move
koa-compose
initialisation higher to avoid checking functions twice (thank you @yujilim)