Perron
A sane client for web services with a built-in circuit-breaker, support for filtering both request and response.
npm install perron --save
Quick Example
The following is a minimal example of using perron
to call a web service:
const ServiceClient = ; // A separate instance of `perron` is required per hostconst catWatch = 'https://catwatch.opensource.zalan.do'; catWatch;
Making Requests
Each call to request
method will return a Promise, that would either resolve to a successful ServiceClient.Response
containing following fields:
statusCode // same as Node IncomingMessage.statusCode headers // same as Node IncomingMessage.headers body // if JSON, then parsed body, otherwise a string
request
method accepts an objects with all of the same properties as https.request method in Node.js, except from a hostname
field, which is taken from the options passed when creating an instance of ServiceClient
. Additionally you can add a timeout
and readTimeout
fields, which define time spans in ms for socket connection and read timeouts.
Handling Errors
For the error case you will get a custom error type ServiceClientError
. A custom type is useful in case you change the request to some other processing in your app and then need to distinguish between your app error and requests errors in a final catch.
ServiceClientError
class contains an optional response
field that is available if any response was received before there was an error.
If you have not specified retry options, you can use instanceof
checks on the error to determine exact reason:
catWatch; { if err instanceof BodyParseError console; console; else if err instanceof RequestFilterError console; else if err instanceof ResponseFilterError console; console; else if err instanceof CircuitOpenError console; else if err instanceof RequestConnectionTimeoutError console; console; else if err instanceof RequestReadTimeoutError console; console; else if err instanceof RequestUserTimeoutError console; console; else if err instanceof RequestNetworkError console; console; else if err instanceof InternalError // This error should not happen during normal operations // and usually indicates a bug in perron or misconfiguration console; }
If you have retries configured, there are only 3 types of errors you will get that are relating to circuit breakers and retries, however you can access original errors that led to retries through retryErrors
field available on both the successful response:
catWatch; { if err instanceof CircuitOpenError console; else if err instanceof ShouldRetryRejectedError console; errretryErrors; else if err instanceof MaximumRetriesReachedError console; errretryErrors; }
Circuit Breaker
It's almost always a good idea to have a circuit breaker around your service calls, and generally one per hostname is also a good default since 5xx usually means something is wrong with the whole service and not a specific endpoint.
This is why perron
by default includes one circuit breaker per instance. Internally perron
uses circuit-breaker-js, so you can use all of it's options when configuring the breaker:
const ServiceClient = ; const catWatch = hostname: 'catwatch.opensource.zalan.do' // If the "circuitBreaker" settings are passed (non-falsy), they will be merged // with the default options below. Otherwise, circuit breaking will be disabled circuitBreaker: windowDuration: 10000 numBuckets: 10 timeoutDuration: 2000 errorThreshold: 50 volumeThreshold: 10 ;
Optionally the onCircuitOpen
and onCircuitClose
functions can be passed to the circuitBreaker object in order to track the state of the circuit breaker via metrics or logging:
const catWatch = hostname: 'catwatch.opensource.zalan.do' circuitBreaker: windowDuration: 10000 numBuckets: 10 timeoutDuration: 2000 errorThreshold: 50 volumeThreshold: 10 { console; } { console; } ;
Circuit breaker will count all errors, including the ones coming from filters, so it's generally better to do pre- and post- validation of your request outside of filter chain.
If this is not the desired behavior, or you are already using a circuit breaker, it's always possible to disable the built-in one:
const catWatch = hostname: 'catwatch.opensource.zalan.do' circuitBreaker: false;
In case if you want perron to still use a circuit breaker but it has to be provided dynamically by your code on-demand you can pass circuitBreaker
option as a function (make sure to not create a circuit breaker for every request):
const catWatch = hostname: 'catwatch.opensource.zalan.do' { return somePreviouslyConstructedCB; };
Retry Logic
For application critical requests it can be a good idea to retry failed requests to the responsible services.
Occasionally target server can have high latency for a short period of time, or in the case of a stack of servers, one server can be having issues and retrying the request will allow perron to attempt to access one of the other servers that currently aren't facing issues.
By default perron
has retry logic implemented, but configured to perform 0 retries. Internally perron
uses node-retry to handle the retry logic and configuration. All of the existing options provided by node-retry
can be passed via configuration options through perron
.
There is a shouldRetry
function which can be defined in any way by the consumer and is used in the try logic to determine whether to attempt the retries or not depending on the type of error and the original request object.
If the function returns true and the number of retries hasn't been exceeded, the request can be retried.
There is also an onRetry
function which can be defined by the user of perron
. This function is called every time a retry request will be triggered.
It is provided the current attempt index, the error that is causing the retry and the original request params.
The first time onRetry
gets called, the value of currentAttempt will be 2. This is because the first initial request is counted as the first attempt, and the first retry attempted will then be the second request.
const ServiceClient = ; const catWatch = hostname: 'catwatch.opensource.zalan.do' retryOptions: retries: 1 factor: 2 minTimeout: 200 maxTimeout: 400 randomize: true { return err && errresponse && errresponsestatusCode >= 500; } { console; } ;
Filters
It's quite often necessary to do some pre- or post-processing of the request. For this purpose perron
implements a concept of filters, that are just an object with 2 optional methods: request
and response
.
By default, every instance of perron
includes a treat5xxAsError
filter, but you can specify which filters should be use by providing a filters
options when constructing an instance. This options expects an array of filter object and is not automatically merged with the default ones, so be sure to use concat
if you want to keep the default filters as well.
There aren't separate request and response filter chains, so given that we have filters A
, B
and C
the request flow will look like this:
A.request ---> B.request ---> C.request ---|
V
HTTP Request
|
A.response <-- B.response <-- C.response <--
If corresponding request
or response
method is missing in the filter, it is skipped, and the flow goes to the next one.
Modifying the Request
Let's say that we want to inject a custom header of the request. This is really easy to do in a request filter:
const ServiceClient = ; // A separate instance of ServiceClient is required per hostconst catWatch = hostname: 'catwatch.opensource.zalan.do' filters: { // let's pretend to be AJAX requestheaders'x-requested-with' = 'XMLHttpRequest'; return request; } ServiceClienttreat5xxAsError;
Resolving Request in a Filter
Sometimes it is necessary to pretend to have called the service without actually doing it. This could be useful for caching, and is also very easy to implement:
const ServiceClient = ; const getCache = ; // A separate instance of ServiceClient is required per hostconst catWatch = hostname: 'catwatch.opensource.zalan.do' filters: { const body = ; if body const headers = {}; const statusCode = 200; return statusCode headers body ; return request; } ServiceClienttreat5xxAsError;
If the request is resolved in such a way, all of the pending filter in the request and response chain will be skipped, so the flow diagram will look like this:
called | not called
---------------------------------
cacheFilter | B.treat5xxAsError
| |
| | HTTP Request
V |
cacheFilter | B.treat5xxAsError
Rejecting Request in a Filter
It is possible to reject the request both in request and response filters by throwing, or by returning a rejected Promise. Doing so will be picked up by the circuit breaker, so this behavior should be reserved by the cases where the service returns 5xx
error, or the response is completely invalid (e.g. invalid JSON).
JSON Parsing
By default Perron will try to parse JSON body if the content-type
header is not set or
it is specified as application/json
. If you wish to disable this behavior you can use
autoParseJson: false
option when constructing ServiceClient
object.
UTF-8 Decoding
By default Perron will try to decode JSON body to UTF-8 string.
If you wish to disable this behaviour, you can use autoDecodeUtf8: false
option
when calling request
method.
Opentracing
Perron accepts a Span like object where it will log the network and request related events.
License
The MIT License
Copyright (c) 2016 Zalando SE
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.