I am using Unicorn as my app server for my Rails app, and am trying to figure out why there sometimes is sometimes a non-trivial (> 5 seconds) delay between the start of a request, and when it reaches my controller.
This is what my production.log prints out:
Started GET "/search/articles.json?q=mashable.com" for 138.7.7.33 at 2015-07-23 14:59:19 -0400**
Parameters: {"q"=>"mashable.com"}
Searching articles for keyword: mashable.com, format: json, Time: 2015-07-23 14:59:26 -0400
Notice how there is a 7 second delay in between STARTED GET: and "Searching articles for keyword", which is the first thing the controller method does.
articles.json is routed to my controller method "articles" which simply does this for now:
def articles
format = params[:format]
keyword = params["q"]
Rails.logger.info "Searching articles for keyword: #{keyword}, format: #{format}, Time: #{Time.now.to_s}"
end
This is my routes.rb
MyApp::Application.routes.draw do
match '/search/articles' => 'search#articles'
#more routes here, but articles is the first route
end
What could possibly cause this delay? Is it because an Unicorn worker is busy? Is it because an Unicorn worker is taking up too much memory which leads the system to be slow?
Note: I don't believe the delay is in making any database connections but I could be wrong. The code doesn't need to make a database call, and the max connections for my database is 1000, and there are usually at most 1-2 connections.
Three thoughts:
You'll probably be better served using Puma instead of Unicorn
It could be that your system is running out of memory, or it could have plenty of memory available: install New Relic to troubleshoot where the bottleneck is
It could also be that you have more Unicorn instances than the number of connections your DB allows, in which case the instance is having to wait for others to disconnect before it can connect. This would likely manifest itself with irregular 5-second delays rather than happening every time.
Actually, it might be caused by an before_filter callback, you should check it
I think it can be because of lack of memory and thus frequent garbage collection, which freeze whole system.
If it's a production problem it could be caused by slow clients sending requests. New Relic and Monit are good options. You could consider sending signals to Unicorn workers to restart them to better understand the problem.
You could also try adding preload_app true in your Unicorn config to speed up the startup time of worker processes.
While adding redis as my cache store in Rails app,
i added the redis url as redis://localhost:6379/0/cache.
What is the meaning of 0/cache in the redis URL?
You may configure redis to use many databases.
The 0 means you are using the first of them (id=0).
The cache is the namespace of the database.
Depending on where you set this url, you are actually telling to use redis caching and values will be saved/retrieved etc from the database 0 and namespace cache.
I sought, but did not find, a max-requests-per-worker option in unicorn similar to gunicorn's max_requests or apache's MaxRequestsPerChild.
Does it exist?
If not, has anyone implemented it?
I'm thinking of putting it in the file where I have oobgc, since that gets control after every requests anyway. Does that sound about right?
The problem is that my unicorn workers are getting big and fat, and garbage collection is taking more and more of my CPU.
i've just released 'unicorn-worker-killer' gem. This enables you to kill Unicorn worker based on 1) Max number of requests and 2) Process memory size (RSS), without affecting the request. It's really easy to use. At first, please add this line to your Gemfile.
gem 'unicorn-worker-killer'
Then, please add the following lines to your config.ru.
# Unicorn self-process killer
require 'unicorn/worker_killer'
# Max requests per worker
use Unicorn::WorkerKiller::MaxRequests, 3072, 4096
# Max memory size (RSS) per worker
use Unicorn::WorkerKiller::Oom, (256*(1024**2)), (384*(1024**2))
It's highly recommended to randomize the threshold to avoid killing all workers at once.
Unicorn doesn't offer a max-requests.
The unicorn master will re-spawn any worker which exits and a worker will gracefully exit at the end of the current request when it receives a QUIT signal, so you could easily roll your own max request logic into your worker request life-cycle.
With Rails, something like the following in your application controller (alternatively, similar logic in a rack middleware)
after_filter do
##request_count ||= 0
Process.kill('QUIT',$$) if (##request_count += 1) > MAX_REQUESTS
end
I just can't find how many apps you can host on heroku with one dyno?
I plan to host a lot of small apps with little traffic.
Thanks for your answers
Dynos are calculated on a per application basis.
However, this doesn't mean you need to buy 3 dynos to run 3 apps.
You can create 3 application each with 1 dyno.
One App per Dyno / subdomain.heroku.com.
Some explanation here: http://docs.heroku.com/performance#backlog-too-deep
I believe you can spin up another web process inside a web dyno. I've done it with workers. One worker dyno had 3 sub-processes. each a copy of the rails app, and each running independently on the database.
How you'd manage to spin up the correct application, I'm not sure... And you'd need a controller application.
I don't want to say it's not possible, because I don't believe that statement is at all constructive. I will say, spawning a new application with a 34$ a month extra dyno fee would be a better use of you time/money.
An additional concern. each web dyno allows for a limited amount of memory, and rails isn't exactly known for being light on memory. When I spawned sub-workers I ran into heaps of memory issues. So many that I eventually rolled the feature out. If I work for an afternoon to try to 'tweak' for the constrains, I've spent more of my bosses money than 4 months of extra dyno's, so I have to weigh it up.
Anyway... Here's how I forked workers
require 'heroku-api'
...
def self.fork_workers(iDesired = 5, iQueue = nil)
cmd = "rake jobs:work WORKER=MY_SERF"
cmd += " QUEUES=#{iQueue}" if(iQueue)
p cmd
if(RUBY_PLATFORM["mingw32"].nil?) #DON'T WORK ON WINDOWS
currentCount = Rush::Box.new.processes.filter(:cmdline => /#{cmd}/ ).size;
iDesired -= currentCount;
if(iDesired > 0)
iDesired.times { Rush::Box.new[Rails.root].bash( cmd, :background => true ) }
elsif(iDesired < 0)
end
end
end
Last note:
One dyno apps will go to sleep if left alone for an hour... Your users will feel the delay during wake up.
https://devcenter.heroku.com/articles/dynos#dyno-idling
Running a rails site right now using SQLite3.
About once every 500 requests or so, I get a
ActiveRecord::StatementInvalid (SQLite3::BusyException: database is locked:...
What's the way to fix this that would be minimally invasive to my code?
I'm using SQLLite at the moment because you can store the DB in source control which makes backing up natural and you can push changes out very quickly. However, it's obviously not really set up for concurrent access. I'll migrate over to MySQL tomorrow morning.
You mentioned that this is a Rails site. Rails allows you to set the SQLite retry timeout in your database.yml config file:
production:
adapter: sqlite3
database: db/mysite_prod.sqlite3
timeout: 10000
The timeout value is specified in miliseconds. Increasing it to 10 or 15 seconds should decrease the number of BusyExceptions you see in your log.
This is just a temporary solution, though. If your site needs true concurrency then you will have to migrate to another db engine.
By default, sqlite returns immediatly with a blocked, busy error if the database is busy and locked. You can ask for it to wait and keep trying for a while before giving up. This usually fixes the problem, unless you do have 1000s of threads accessing your db, when I agree sqlite would be inappropriate.
// set SQLite to wait and retry for up to 100ms if database locked
sqlite3_busy_timeout( db, 100 );
All of these things are true, but it doesn't answer the question, which is likely: why does my Rails app occasionally raise a SQLite3::BusyException in production?
#Shalmanese: what is the production hosting environment like? Is it on a shared host? Is the directory that contains the sqlite database on an NFS share? (Likely, on a shared host).
This problem likely has to do with the phenomena of file locking w/ NFS shares and SQLite's lack of concurrency.
If you have this issue but increasing the timeout does not change anything, you might have another concurrency issue with transactions, here is it in summary:
Begin a transaction (aquires a SHARED lock)
Read some data from DB (we are still using the SHARED lock)
Meanwhile, another process starts a transaction and write data (acquiring the RESERVED lock).
Then you try to write, you are now trying to request the RESERVED lock
SQLite raises the SQLITE_BUSY exception immediately (indenpendently of your timeout) because your previous reads may no longer be accurate by the time it can get the RESERVED lock.
One way to fix this is to patch the active_record sqlite adapter to aquire a RESERVED lock directly at the begining of the transaction by padding the :immediate option to the driver. This will decrease performance a bit, but at least all your transactions will honor your timeout and occurs one after the other. Here is how to do this using prepend (Ruby 2.0+) put this in a initializer:
module SqliteTransactionFix
def begin_db_transaction
log('begin immediate transaction', nil) { #connection.transaction(:immediate) }
end
end
module ActiveRecord
module ConnectionAdapters
class SQLiteAdapter < AbstractAdapter
prepend SqliteTransactionFix
end
end
end
Read more here: https://rails.lighthouseapp.com/projects/8994/tickets/5941-sqlite3busyexceptions-are-raised-immediately-in-some-cases-despite-setting-sqlite3_busy_timeout
Just for the record. In one application with Rails 2.3.8 we found out that Rails was ignoring the "timeout" option Rifkin Habsburg suggested.
After some more investigation we found a possibly related bug in Rails dev: http://dev.rubyonrails.org/ticket/8811. And after some more investigation we found the solution (tested with Rails 2.3.8):
Edit this ActiveRecord file: activerecord-2.3.8/lib/active_record/connection_adapters/sqlite_adapter.rb
Replace this:
def begin_db_transaction #:nodoc:
catch_schema_changes { #connection.transaction }
end
with
def begin_db_transaction #:nodoc:
catch_schema_changes { #connection.transaction(:immediate) }
end
And that's all! We haven't noticed a performance drop and now the app supports many more petitions without breaking (it waits for the timeout). Sqlite is nice!
bundle exec rake db:reset
It worked for me it will reset and show the pending migration.
Sqlite can allow other processes to wait until the current one finished.
I use this line to connect when I know I may have multiple processes trying to access the Sqlite DB:
conn = sqlite3.connect('filename', isolation_level = 'exclusive')
According to the Python Sqlite Documentation:
You can control which kind of BEGIN
statements pysqlite implicitly
executes (or none at all) via the
isolation_level parameter to the
connect() call, or via the
isolation_level property of
connections.
I had a similar problem with rake db:migrate. Issue was that the working directory was on a SMB share.
I fixed it by copying the folder over to my local machine.
Most answers are for Rails rather than raw ruby, and OPs question IS for rails, which is fine. :)
So I just want to leave this solution over here should any raw ruby user have this problem, and is not using a yml configuration.
After instancing the connection, you can set it like this:
db = SQLite3::Database.new "#{path_to_your_db}/your_file.db"
db.busy_timeout=(15000) # in ms, meaning it will retry for 15 seconds before it raises an exception.
#This can be any number you want. Default value is 0.
Source: this link
- Open the database
db = sqlite3.open("filename")
-- Ten attempts are made to proceed, if the database is locked
function my_busy_handler(attempts_made)
if attempts_made < 10 then
return true
else
return false
end
end
-- Set the new busy handler
db:set_busy_handler(my_busy_handler)
-- Use the database
db:exec(...)
What table is being accessed when the lock is encountered?
Do you have long-running transactions?
Can you figure out which requests were still being processed when the lock was encountered?
Argh - the bane of my existence over the last week. Sqlite3 locks the db file when any process writes to the database. IE any UPDATE/INSERT type query (also select count(*) for some reason). However, it handles multiple reads just fine.
So, I finally got frustrated enough to write my own thread locking code around the database calls. By ensuring that the application can only have one thread writing to the database at any point, I was able to scale to 1000's of threads.
And yea, its slow as hell. But its also fast enough and correct, which is a nice property to have.
I found a deadlock on sqlite3 ruby extension and fix it here: have a go with it and see if this fixes ur problem.
https://github.com/dxj19831029/sqlite3-ruby
I opened a pull request, no response from them anymore.
Anyway, some busy exception is expected as described in sqlite3 itself.
Be aware with this condition: sqlite busy
The presence of a busy handler does not guarantee that it will be invoked when there is
lock contention. If SQLite determines that invoking the busy handler could result in a
deadlock, it will go ahead and return SQLITE_BUSY or SQLITE_IOERR_BLOCKED instead of
invoking the busy handler. Consider a scenario where one process is holding a read lock
that it is trying to promote to a reserved lock and a second process is holding a reserved
lock that it is trying to promote to an exclusive lock. The first process cannot proceed
because it is blocked by the second and the second process cannot proceed because it is
blocked by the first. If both processes invoke the busy handlers, neither will make any
progress. Therefore, SQLite returns SQLITE_BUSY for the first process, hoping that this
will induce the first process to release its read lock and allow the second process to
proceed.
If you meet this condition, timeout isn't valid anymore. To avoid it, don't put select inside begin/commit. or use exclusive lock for begin/commit.
Hope this helps. :)
this is often a consecutive fault of multiple processes accessing the same database, i.e. if the "allow only one instance" flag was not set in RubyMine
Try running the following, it may help:
ActiveRecord::Base.connection.execute("BEGIN TRANSACTION; END;")
From: Ruby: SQLite3::BusyException: database is locked:
This may clear up the any transaction holding up the system
I believe this happens when a transaction times out. You really should be using a "real" database. Something like Drizzle, or MySQL. Any reason why you prefer SQLite over the two prior options?