In high volume applications it can be very difficult to figure out what is happening in logs because each request is not easily identified. For example 3 requests could look something like this:

```
Started GET "/" for 72.48.77.213 at 2016-01-06 20:30:21 +0000
  Rendered welcome/index.html.erb within layouts/application (0.1ms)
Started GET "/" for 72.48.77.213 at 2016-01-06 20:30:22 +0000
Started GET "/" for 72.48.77.213 at 2016-01-06 20:30:23 +0000
  Rendered welcome/index.html.erb within layouts/application (0.1ms)
Processing by WelcomeController#index as HTML
Completed 200 OK in 5ms (Views: 3.8ms | ActiveRecord: 0.0ms)
Processing by WelcomeController#index as HTML
  Rendered welcome/index.html.erb within layouts/application (0.1ms)
Completed 200 OK in 5ms (Views: 3.8ms | ActiveRecord: 0.0ms)
  Processing by WelcomeController#index as HTML
Completed 200 OK in 5ms (Views: 3.8ms | ActiveRecord: 0.0ms)
```

The `:request_id` log tag ensures that each request is tagged with a unique identifier. While they are still interleaved it is possible to figure out which lines belong to which requests. Like:

```
[c6034478-4026-4ded-9e3c-088c76d056f1] Started GET "/" for 72.48.77.213 at 2016-01-06 20:30:21 +0000
[c6034478-4026-4ded-9e3c-088c76d056f1]  Rendered welcome/index.html.erb within layouts/application (0.1ms)
[abuqw781-5026-6ded-7e2v-788c7md0L6fQ] Started GET "/" for 72.48.77.213 at 2016-01-06 20:30:22 +0000
[acfab2a7-f1b7-4e15-8bf6-cdaa008d102c] Started GET "/" for 72.48.77.213 at 2016-01-06 20:30:23 +0000
[abuqw781-5026-6ded-7e2v-788c7md0L6fQ]  Rendered welcome/index.html.erb within layouts/application (0.1ms)
[c6034478-4026-4ded-9e3c-088c76d056f1] Processing by WelcomeController#index as HTML
[c6034478-4026-4ded-9e3c-088c76d056f1] Completed 200 OK in 5ms (Views: 3.8ms | ActiveRecord: 0.0ms)
[abuqw781-5026-6ded-7e2v-788c7md0L6fQ] Processing by WelcomeController#index as HTML
[abuqw781-5026-6ded-7e2v-788c7md0L6fQ]  Rendered welcome/index.html.erb within layouts/application (0.1ms)
[abuqw781-5026-6ded-7e2v-788c7md0L6fQ] Completed 200 OK in 5ms (Views: 3.8ms | ActiveRecord: 0.0ms)
[acfab2a7-f1b7-4e15-8bf6-cdaa008d102c]  Processing by WelcomeController#index as HTML
[acfab2a7-f1b7-4e15-8bf6-cdaa008d102c] Completed 200 OK in 5ms (Views: 3.8ms | ActiveRecord: 0.0ms)
```

Now if you have the logs and you find this unique ID you can filter to only look at information from that request. So a filtered log output would be very clear:

```
[c6034478-4026-4ded-9e3c-088c76d056f1] Started GET "/" for 72.48.77.213 at 2016-01-06 20:30:21 +0000
[c6034478-4026-4ded-9e3c-088c76d056f1]  Rendered welcome/index.html.erb within layouts/application (0.1ms)
[c6034478-4026-4ded-9e3c-088c76d056f1] Processing by WelcomeController#index as HTML
[c6034478-4026-4ded-9e3c-088c76d056f1] Completed 200 OK in 5ms (Views: 3.8ms | ActiveRecord: 0.0ms)
```

In addition to this benefit the `:request_id` can be set via the `X-Request-ID` header so that the same request could be traced between multiple components. For example a request comes in Nginx (or another load balancer) could assign a request id. As the load balancer processes the request I can log using that id, then when the request is passed on to Rails, the same id is used. That way if a problem is determined to be not caused in Rails it could be traced back to other components with the same ID. You can set a value in nginx for example using something like this:

```
  location / {
      proxy_pass http://upstream;
      proxy_set_header X-Request-Id $pid-$msec-$remote_addr-$request_length;
  }
# http://stackoverflow.com/questions/17748735/setting-a-trace-id-in-nginx-load-balancer
```

Heroku sets this header value so problems like timeouts that are logged by like router can be traced back to specific request within the application.

Whether you are using components that are setting request ID or not, all production applications can benefit from the additional debugging information of having a unique identifier for all requests. This change will only affect new applications, anyone can opt out by commenting or removing the line in `config/production.rb`.

Refactor `connection.insert_sql`

`connection.insert_sql` is almost the same as `connection.insert`.

[close #22917] Don't output to `STDOUT` twice

When `rails console` or `rails server` are used along with a logger set to output to `STDOUT` then the contents will show up twice. This happens because the logger is extended with `ActiveSupportLogger.broadcast` with a destination of STDOUT even if it is already outputting to `STDOUT`.

Previously PR #22592 attempted to fix this issue, but it ended up causing NoMethodErrors. A better approach than relying on adding a method and flow control is to inspect the log destination directly. For this `ActiveSupport::Logger.logger_outputs_to?` was introduced

```ruby
logger = Logger.new(STDOUT)
ActiveSupport::Logger.logger_outputs_to?(logger, STDOUT)
# => true
```

To accomplish this we must look inside of an instance variable of standard lib's Logger `@logdev`. There is a related Ruby proposal to expose this method in a standard way: https://bugs.ruby-lang.org/issues/11955

Fix remove_index for postgresql when running legacy migrations

remove activemodel dependency on builder

make generated controller test work correctly

Since the `#file_name` that not consideration for the namespace, if generate a controller with a namespace,
not the correct url helper generation, it had become an error to run the test.

Modified to generate the correct url helper, even if it is produced a namespace with controller.

Autoload ReferenceDefinition class in abstract adapter so that it can be used by #add_reference in schema_statements

Add short-hand methods for text and blob types in MySQL

Per-form CSRF tokens

Provide a better error message if a user mistypes the name of script …

Add tests for detecting bad options for runner

While the type definition is in Active Model the change of behavior will
be only user facing in Active Record so better to put the entry in its
changelog.

[ci skip]

Take UTC offset into account when assigning string value to time attributes.

Fix `connection#create` in PG adapter

This reverts related commits due to #22917.

This fixes the case when you try to render an html you know safe and the
file is named something.html. With this commit the content of the html
won't be escaped anymore because AV won't use Raw handler and choose
Html handler instead.

[ci skip] Update doc/resource link removed by #21211

[ci skip] Add job suffix to sample's job file name

Autoload ReferenceDefinition class in abstract adapter so that it can be used by #add_reference in schema_statements

- Fixes #22916.

In Pg and Sqlite3, `:text` and `:binary` have variable unlimited length.
But in MySQL, these have limited length for each types (ref #21591, #21619).
This change adds short-hand methods for each text and blob types.

Example:

    create_table :foos do |t|
      t.tinyblob   :tiny_blob
      t.mediumblob :medium_blob
      t.longblob   :long_blob
      t.tinytext   :tiny_text
      t.mediumtext :medium_text
      t.longtext   :long_text
    end

Originally `connection#create` had aliased to `connection#insert` in PG
adapter. But it was broken by #7447. Re-alias `create` to `insert` for
fixing it.

Fix `unsigned?` and `blob_or_text_column` for Enum columns in MySQL

Add ActionController:Renderers test

activerecord: allocate fewer objects

Fix user name in doc [ci skip]

Allow add_to_transaction with null transaction