I have handlers that respond to https requests. In the handlers I call a function F1() which does some application logic and connects to a mysql db and does a query. I want to know how I can use the golang context package to cancel the Db query if the client cancels the request. Do I need to pass the ctx to F1()? Also the code I have now will take 4 seconds even if F1() returns in less then 4. How can I return as soon as F1() returns?
func handler(w http.ResponseWriter, r *http.Request) {
ctx:= r.context()
F1()
select {
case <-ctx.Done():
case <- time.After( 4*time.Second):
}
w.WriteHeader(http.statusOk)
return
}
To begin, I highly recommend taking a look at the Context blog post to familiarize yourself with contexts, in addition to reading over the context documentation itself.
To address your specific questions:
How can you cancel the database query if the user cancels their quest?
To make this work, there are a few things you want to check:
Ensure that your database driver (if you are using database/sql) supports context cancellation.
Ensure you are using the Context variants of all available methods (e.g. db.QueryContext instead of db.Query).
Ensure that you are passing the context (or a derivative of the context) through the stack from your HTTP request through to the database calls.
Do I need to pass the ctx to F1()?
Per #3 above, yes: you will need to pass the context through all intermediate calls to "connect" the database call with the request context.
How can I return as soon as F1() returns?
The code that you have in your question calls F1 in series, rather than concurrently, with your cancellation/timeout select.
If you want to apply a specific deadline to your database call, use context.WithTimeout to limit how long it can take. Otherwise, you do not need to do anything special: just call F1 with your context, and the rest will happen for you, no select is needed.
Related
For the past few days, I've been reading about Go and one concept that I keep returning to are contexts.
I think I understand the motivation behind creating such a structure. The thing that I don't understand is a particular use case when using a context in the incoming HTTP request.
Let's say we have a following httpHandlerFunc. Inside that handler, we call a function that requires a context to be passed. I often saw this solution
func myHandler(w http.ResponseWriter, r *http.Request) {
ctx := context.WithValue(context.Background(), "request", r)
otherFunc(ctx)
}
My question is, why don't we just pass a context from the request, like so
func myHandler(w http.ResponseWriter, r *http.Request) {
otherFunc(r.Context())
}
Doesn't it make more sense to pass the context of the request since we want the context to flow through our program? I thought that creating a background context is something we want to do only in the root parent, like init() function.
You might be missing the main point of contexts — supposedly due to poor HOWTOs you're dealing with.
The possiblility of carrying around arbitrary values in contexts is actually a misfeature of this type, regretted by its designers because it creates an anti-pattern (a proper way to deal with context-as-some-state is to have a set of values explicitly passed around).
The chief reason contexts exist is because they provide tree-like propagation of a signal (cancellation or "done" in the case of contexts).
So the original idea behind contexts is like follows:
The "root" context object is created for an incoming request.
Each "task" which is needed to be executed on behalf of the request is associated with its own context, derived from that of the request¹.
Those tasks may produce other tasks and so on.
As you can see, a hierarchy of "units of works" is formed, — linked to the object which is the reason for these units to exist and execute.
When the incoming request is cancelled (the client's socket got disconnected, for example), the context object associated with it is cancelled as well, and then all the linked tasks receive it as it's propagated from the root of the resulting context tree down to its leaves — making sure all the tasks being executed for the request are (eventually) cancelled.
Of course, in order for this to work, each "task" — which is usually a goroutine doing something — is required to "listen" from the context passed to it for that "done" signal.
Contexts also support timeout out of the box, so you might create a context which cancels itself after some fixed time interval passes.
So, back to the examples in your question.
The first example ignores the request's context completely and creates a from-scratch context ostensibly with the sole reason to carry stuff in it (bad).
The second example might use the context for its intended purpose (but we do not know as we cannot see that otherFunc).
I would advise you to read https://blog.golang.org/context, and the articles on concurrency patters in Go linked there.
¹ Actually, a new context need not be created if the task to be controlled by it has no other policy to "add" to the existing, parent, context.
The idea of derivation is here to implment additional ways to cancel work in this particular task as well as honoring the cancellation of the parent context.
For instance, a context derived for a particular task could have its own deadline or have a way to cancel only this particular context.
Of course, a complex—nested—context can be derived for a task: for example, a context with a deadline can be derived from the parent context, and then a cancellable context can be derived form the former. The result would be a context which is cancelled either explicitly by the code or when the deadline expires or when the parent context signals its cancellation.
Your two examples do entirely different things.
func myHandler(w http.ResponseWriter, r *http.Request) {
ctx := context.WithValue(context.Background(), "request", r)
otherFunc(ctx)
}
This creates a new context, and stores the request as a value. There is rarely, if ever, any reason to do exactly this. A far more idiomatic solution would be just to pass the request to otherFunc like so:
func myHandler(w http.ResponseWriter, r *http.Request) {
otherFunc(r)
}
If you really do need to pass the request as a context value, you should probably do it with the current request's context, like so:
func myHandler(w http.ResponseWriter, r *http.Request) {
ctx := context.WithValue(r.Context(), "request", r)
otherFunc(ctx)
}
I want to run some slow routine in another goroutine, is it safe to do it like this:
func someHandler(w http.ResponseWriter, r *http.Request) {
go someReallySlowFunction() // sending mail or something slow
fmt.Fprintf(w,"Mail will be delivered shortly..")
}
func otherHandler(w http.ResponseWriter, r *http.Request) {
foo := int64(0)
bar := func() {
// do slow things with foo
}
go bar()
fmt.Fprintf(w,"Mail will be delivered shortly..")
}
Is there any gotchas by doing this?
Serving each http request runs in its own goroutine (more details on this). You are allowed to start new goroutines from your handler, and they will run concurrently, independently from the goroutine executing the handler.
Some things to look out for:
The new goroutine runs independently from the handler goroutine. This means it may complete before or after the handler goroutine, you cannot (should not) assume anything regarding to this without explicit synchronization.
The http.ResponseWriter and http.Request arguments of the handler are only valid and safe to use until the handler returns! These values (or "parts" of them) may be reused - this is an implementation detail of which you should also not assume anything. Once the handler returns, you should not touch (not even read) these values.
Once the handler returns, the response is committed (or may be committed at any moment). Which means your new goroutine should not attempt to send back any data using the http.ResponseWriter after this. This is true to the extent that even if you don't touch the http.ResponseWriter in your handler, not panicing from the handler is taken as a successful handling of the request and thus HTTP 200 status is sent back (see an example of this).
You are allowed to pass the http.Request and http.ResponseWriter values to other functions and to new goroutines, but care must be taken: you should use explicit synchronization (e.g. locks, channels) if you intend to read / modify these values from multiple goroutines (or you want to send back data from multiple goroutines).
Note that seemingly if both your handler goroutine and your new goroutine just reads / inspects the http.Request, that still may be problematic. Yes, multiple goroutines can read the same variable without synchronization (if nobody modifies it). But calling certain methods of http.Request also modify the http.Request, and without synchronization there is no guarantee what other goroutines would see from this change. For example Request.FormValue() returns a form value associated with the given key. But this method calls ParseMultiPartForm() and ParseForm() if necessary which modify the http.Request (e.g. they set the Request.PostForm and Request.Form struct fields).
So unless you synchronize your goroutines, you should not pass Request and ResponseWriter to the new goroutine, but acquire data needed from the Request in the handler goroutine, and pass only e.g. a struct holding the needed data.
Your second example:
foo := int64(0)
bar := func() {
// do slow things with foo
}
go bar()
This is perfectly fine. This is a closure, and local variables referred by it will survive as long as they are accessible.
Note that alternatively you could pass the value of the local variable to the anonymous function call as an argument like this:
foo := int64(0)
bar := func(foo int64) {
// do slow things with param foo (not the local foo var)
}
go bar(foo)
In this example the anonymous function will see and use its parameter foo and not the local variable foo. This may or may not be what you want (depending on whether the handler also uses the foo and whether changes made by any of the goroutines need to be visible to the other - but that would require synchronization anyway, which would be superseded by a channel solution).
If you care for acknowledgement for the mail, then the posted code won't help. Running the code in separate goroutine makes it independent and the server reply will be success even if the mail is not sent due to some error in the goroutine function.
Im creating a Networking API and want people to be able to route requests to specific endpoints using a ServeMux. Instead of using a Server instance, I need to use my own low level ServerConn. This is because I am receiving both incoming HTTP requests and plain text data from the same port.
The problem, however, is that if I want to forward a request using my ServeMux, I would use it's ServeHTTP method. For this, I need to provide a ResponseWriter, which I don't know how to create an instance of since it is an interface, not a struct. Should a I create my own ResponseWriter struct? Is there one given by the Golang Standard Library? Or is there an alternate solution to this altogether?
I would avoid doing this altogether if at all possible. Mixing protocols on the same connection is bound to lead to hard-to-trace bugs, and unexpected behavior. If you really want to do it, and have all the http/1.1 mechanisms work correctly, leave as much as possible to the http package.
Since ResponseWriter is an interface, you would implement your own type to satisfy it. Look at the unexported response type in the http package for a full example. There's a lot to get right, and using it in combination with a ServerConn (which is documented as "do no use") is probably not a good idea.
The place to do this at a lower level would be in Accept inside the Server's net.Listener. Since you're going to have to parse the start of every request twice, you would need a net.Conn that can be "rewound" partly.
Make yourself a net.Listener that checks the start of the stream on a new connection, and if it looks like an http request, return a net.Conn that replays the first chunk you read off the wire on its first Reads. Something like:
type replayConn struct {
net.Conn
buf []byte
pos int
}
func (c *replayConn) Read(b []byte) (int, error) {
if c.pos < len(c.buf) {
n := copy(b, c.buf[c.pos:])
c.pos += n
return n, nil
}
return c.Conn.Read(b)
}
If the connection isn't http, then send the connection off to your other type of handler, and continue blocking on Accept.
I'm looking to pull some repetitive logic out of my handlers and put it into some per-handler middleware: specifically things like CSRF checks, checking for an existing session value (i.e. for auth, or for preview pages), etc.
I've read a few articles on this, but many examples focus on a per-server middleware (wrapping http.Handler): I have a smaller set of handlers that need the middleware. Most of my other pages do not, and therefore if I can avoid checking sessions/etc. for those requests the better.
My middleware, so far, typically looks something like this:
func checkCSRF(h http.HandlerFunc) http.HandlerFunc {
return func(w http.ResponseWriter, r *http.Request) {
// get the session, check/validate/create the token based on HTTP method, etc.
// return HTTP 403 on a failed check
// else invoke the wrapped handler h(w, r)
}
}
However, in many cases I want to pass a variable to the wrapped handler: a generated CSRF token to pass to the template, or a struct that contains form data—one piece of middleware checks the session for the presence of some saved form data before the user hits a /preview/ URL, else it redirects them away (since they have nothing to preview!).
I'd like to pass that struct along to the wrapped handler to save having to duplicate the session.Get/type assertion/error checking logic I just wrote in the middleware.
I could write something like:
type CSRFHandlerFunc func(w http.ResponseWriter, r *http.Request, t string)
... and then write the middleware like so:
func csrfCheck(h CSRFHandlerFunc) http.HandlerFunc {
return func(w http.ResponseWriter, r *http.Request) {
// get the session, check/validate/create the/a token based on HTTP method, etc.
// return HTTP 403 on a failed check
// else invoke the wrapped handler and pass the token h(w, r, token)
}
... but that raises a few questions:
Is this a sensible way to implement per-handler middleware and pass per-request variables?
Prior to testing this (don't have access to my dev machine!), if I need to wrap a handler with multiple pieces of middleware, I assume I can just r.HandleFunc("/path/preview/", checkCSRF(checkExisting(previewHandler)))? The issue I'm seeing here is that the middleware is now tightly coupled: the wrapped middleware now needs to receive and then pass on the variable from the outer middleware. This makes extending http.HandlerFunc trickier/more convoluted.
Would gorilla/context fit better here and allow me to avoid writing 2-3 custom handler types (or a generic handler type) — and if so, how would I make use of it? Or could I implement my own "context" map (and run into issues with concurrent access?).
Where possible I'm trying to avoid falling for the "don't get caught writing a library" trap, but middleware is something that I'm likely to add/build on later in the project's life, and I'd like to "get it right" the first time around.
Some guidance on this would be much appreciated. Go's been great so far for writing a web application, but there's not a ton of examples around at this stage in its life and I'm therefore leaning on SO a little.
If I understood your question correctly, you're looking for a convenient way to pass additional parameters to your middleware, right?
Now, it's important to define what those parameters are. They could be some configuration values for your middleware – those can be set when the Handler type is being constructed). Instead of NewMyMiddleware(MyHandler), you do NewMyMiddleware(MyHandler, "parameter"), no problem here.
But in your case it seems like you want to pass per-request parameters, like a CSRF token. Passing those into the handler function would modify its signature and it would deviate from the standard Handler[Func] interface. You're right about middleware being more tightly coupled in this case.
You kind of mentioned the solution yourself – a context map is, in my opinion, a viable tool for this. It's not that hard to write one yourself – you basically need a map[*http.Request]interface{} and an RWMutex for safe concurrent access. Still, simply using gorilla/context should suffice – it seems like a (relatively) mature, well-written package with a nice API.
Shameless plug: if you're dealing with CSRF checks, why not try out my nosurf package?
The problem
One data source generating data in format {key, value}
Multiple receivers each waiting for different key
Example
Getting data is run in loop. Sometimes I will want to get next value labelled with key by using
Value = MyClass:GetNextValue(Key)
I want my code to stop there until the value is ready (making some sort of future(?) value). I've tried using simple coroutines, but they work only when waiting for any data.
So the question I want to ask is something like How to implement async values in lua using coroutines or similar concept (without threads)?
Side notes
The main processing function will, apart from returning values to waiting consumers, process some of incoming data (say, labeled with special key) itself.
The full usage context should look something like:
-- in loop
ReceiveData()
ProcessSpecialData()
--
-- Called outside the loop:
V = RequestDataWithGivenKey(Key)
How to implement async values
You start by not implementing async values. You implement async functions: you don't get the value back until has been retrieved.
First, your code must be in a Lua coroutine. I'll assume you understand the care and feeding of coroutines. I'll focus on how to implement RequestDataWithGivenKey:
function RequestDataWithGivenKey(key)
local request = FunctionThatStartsAsyncGetting(key)
if(not request:IsComplete()) then
coroutine.yield()
end
--Request is complete. Return the value.
return request:GetReturnedValue()
end
FunctionThatStartsAsyncGetting returns a request back to the function. The request is an object that stores all of the data needs to process the specific request. It represents asking for the value. This should be a C-function that starts the actual async getting.
The request will be either a userdata or an encapsulated Lua table that stores enough information to communicate with the C-code that's doing the async fetching. IsComplete uses the internal request data to see if that request has completed. GetReturnedValue can only be called when IsComplete returns true; it puts the value on the Lua stack, so that this function can return it.
Your external code simply needs to handle the async stuff internally. Between resumes of these Lua coroutines, you'll need to pump whatever async stuff is doing the fetching, if there are outstanding requests.