At AppSignal we use a lot of technical terms. Read on to learn more about the language of AppSignal.


Actions are the location in the code that a HTTP request, background job or other, code execution in the app itself is started as determined by the parent framework. For Ruby on Rails Rails and Elixir Phoenix apps for example this is the controller and action combination, such as BlogPostsController#create or Api::UsersController#index.

Error incidents also support errors without an action, as an error can occur outside of an action, such as in the app's framework before reaching the app's own code. Performance incidents without an action name set on the transaction are ignored.

Actions, along with the namespace, and for errors the error type, combined create the uniqueness marker for an incident. A different error that occurs on the same action in the same namespace as another incident, creates a new incident for that error.


AppSignal uses an "agent" to communicate with the AppSignal servers and instrument the hosts an application is running on. The host data used in our host metrics feature.

The agent is started by the language specific library and runs as a separate UNIX process on the application's host. The language library and agent communicate with each other through a UNIX socket using an extension.

The transaction instrumentation data collected by the agent is sent to the AppSignal servers. The transaction data is processed and used to detect events worth alerting users about.

Read more about how the AppSignal agent operates and the life cycle of an AppSignal request.


During an HTTP request or a background job an application uses memory. Every data structure that's loaded and object that's instantiated takes up memory. When a request or job handles a lot of data it's possible a lot more memory is used than normally.

The AppSignal integration libraries keep track of allocations per transaction. The integration also keeps track what part of the application allocates more memory objects than other, so it's possible to see if the application's ORM or template library was taking up more memory.

On AppSignal.com the number of allocations is displayed in an Integer for an action, event groups and for specific events. This allocation number is the number of objects have been created in memory during an action/group/event. The size of the created object is not tracked.

Anomaly detection

Anomaly detection is an AppSignal feature that allows users to detect abnormalities in their apps. These are events such as: an increased error rate or a limited amount of free memory on the app's host.

Anomaly detection works with Triggers threshold conditions that create Alerts when their threshold is reached. When an Alert is opened, AppSignal will send notifications to whichever notifiers have been selected for the specific trigger.

See our documentation about Anomaly detection for more information.


An API is an "Application Programming Interface". The term API is a very broad and is used in many ways. In the context of our documentation we refer to the AppSignal HTTP REST API when using "API".

The AppSignal API is a HTTP REST API which can be used to retrieve data from the AppSignal servers and augment the existing data with more information such as Markers.

The API differs from the Push API in its purpose. While the Push API is used to send data to AppSignal by the agent, the HTTP REST API is primarily used to retrieve data from AppSignal. This allow users to use the available data in other tooling.

Read more about our API.

API key

In order to connect with the AppSignal API it's necessary to authenticate yourself. Every user has their own API key to authenticate with AppSignal.

Your own API key can be found on your user profile.

This key is not to be confused with the Push API key which is used by applications with an AppSignal agent installed.


Applications (previously known as "sites", also referred to as "apps") are Ruby and Elixir applications monitored by AppSignal.

After installing the AppSignal library in an application AppSignal starts monitoring these applications. Once we receive data from an application, we register it using the application name and environment it's running in.

An application can have multiple environments as long as every environment uses the same name.

  • "Demo application" - Development
  • "Demo application" - Production
  • "Demo application" - Staging
  • "Demo application" - Test

Multiple applications can exist in one organization.

Read more about applications in our Applications documentation.

AppSignal vs Appsignal vs appsignal

Three different capitalized versions of the AppSignal name?

Yes.. Let's explain!


We have a blog at blog.appsignal.com!

On our blog we post all things relevant to AppSignal, such as new features, new major agent releases, Ruby Magic articles and other awesome stories from our internal process. Such as how to eat stroopwafels.


We keep a full product changelog at appsignal.com/changelog.

All our new features, agent releases and other changes to the product can be found in this neatly organized list.

We also have a small indicator in our top navigation that notifies you if there's a new entry to the changelog.


The configuration is part of the libraries running in applications. It tells the AppSignal libraries what to instrument in an application, which application it is and in which environment it's running.

The AppSignal libraries have multiple methods of configuration. The most common method of configuration is the usage of an appsignal.yml configuration file. The usage of environment variables is also recommended.

For the configuration of the Ruby agent we recommend you read the configuration topic to get started.

CPU usage

During the operation of an application the CPU usage can vary wildly. Some operations of an application can request more CPU time than others.

Other factors can also affect the CPU usage. If the monitored application is not the only process on the host machine other processes can also affect the CPU usage metric. For example, if a database running on the same machine has to perform a complicated query it will request more CPU time.

On AppSignal.com the CPU usage of an application is displayed in two ways. For an action where an error/performance issue occurred and for host metrics. This way it's possible to see if the performance of an action was directly affected by a busy CPU or if the entire host was affected for longer periods of time.

Learn more about what CPU metrics mean on our blog.

Cross-Site Request Forgery

Cross-Site Request Forgery (CSRF) is an attack that forces an end user to execute unwanted actions on a web application where they are authenticated.

A victim may click on a link sent to them via email or web chat, allowing the attacker to steal and use the victim's credentials. The severity of the attack can vary depending on the application and authorization access of the victim's account. An attacker may be able to make state-altering actions to a user's account, such as changing their login credentials, and can compromise an entire application in severe cases, such as when a victim is an administrator.

Elixir Alchemy

Want to learn more about Elixir? In our email series Elixir Alchemy (1x p/mo) we dive deeper into Elixir and Erlang. You can sign up here: appsignal.com/elixir-alchemy.


Most application can be run in different modes. During development of an application other rules apply and errors are not usually shown in the form of a "500 internal server error" page like they do in "production".

AppSignal also understands the concept of an environment allowing different settings be configured per environment. For every environment a separate application is created on AppSignal.com to be configured with its own set of alerting rules and third-party integrations.


Errors are problems that occur during the runtime of an application. This can be anything ranging from a catastrophic failure causing the application to crash or a simple typo causing an error page.

Once an error occurs in an application AppSignal sends an alerts and records the details of the error for later viewing on AppSignal.com.

Read more about the errors feature on our tour page. To learn more about error handling in Ruby, read our Exception handling topic describing how to effectively track errors with AppSignal.


An event is something that happened. This is a very vague statement, because the concept of an event is something very high-level. An error is an event, and so is an performance issue.

AppSignal monitors many events inside an application an on a server using host metrics. By collecting many different types of events and combining the data we hope to provide an as complete as possible picture to gain more insights in applications.

Also see instrumentation events.


The AppSignal libraries and agent are in constant communication with each other. The libraries send data to the agent over a UNIX socket. To do so the libraries use an extension for the programming language they're written in. This extension is written in C and Rust, and installed when the language specific agent is installed.

Read more about the AppSignal extension operates and the life cycle of an AppSignal request.


The impact of an action on an application is based on its usage compared to other actions. When one controller action or job takes more time or is executed more often than others its impact grows.

For example:

  • Action A is triggered 1000 times with an average duration of 0.5 seconds. The combined duration is 500 seconds.
  • Action B is triggered 500 times with an average duration of 3 seconds. The combined duration is 1500 seconds.

Total combined duration of both actions is 2000 seconds.

  • Action A has an impact of 25% (500 / 2000).
  • Action B has an impact of 75% (1500 / 2000).


Instrumentation is what powers AppSignal. Without it, we wouldn't know anything about what's going on inside Applications.

Instrumentation are hooks inside or around frameworks and libraries AppSignal uses to monitor application code. This instrumentation reports errors and slow code which is sent to AppSignal for analysis. Once a problem is detected an alert is sent.

Learn more about Ruby instrumentation.

Instrumentation events

With any kind of code there are multiple levels of code at work. Databases are accessed, JSON is parsed, views are rendered, etc. All these pieces of logic are separate events picked up by the AppSignal instrumentation.

These individual events make it possible to see which parts of the code are slower than others and by how much exactly.

Also see instrumentation.


AppSignal uses language specific libraries to monitor applications. Currently we have a Ruby gem and Elixir package. These libraries include hooks into frameworks and libraries to instrument code blocks such as database calls, file system calls and view rendering.

Every library is specialized in instrumentation of its subject language. Most AppSignal libraries also includes an "agent" which the libraries use to communicate with the AppSignal servers.

Read more about how the AppSignal agent operates and the life cycle of an AppSignal request.

Library integrations

AppSignal libraries integrate with a variety of different libraries and frameworks specific for the programming language. The Ruby gem integrates with pure Ruby, Rails, Sinatra and other available frameworks. The Elixir package integrates with pure Elixir and Phoenix.

These automatic integrations make it easier to get more insight in applications without having to add AppSignal instrumentation manually.

Read more about what Ruby integrations we offer.


Markers are little icons used in graphs on AppSignal.com to indicate a change. This can be a code deploy using a "Deploy marker" or a special event with a "Custom marker". These custom markers can be anything from scaling operations, sudden spikes in traffic or when a database was acting up.

Read more about markers.


Metadata is data that gives information about other data.

Metadata is information about an error or performance issue that provides more context to the data collected. By sending extra metadata on a sample it's easier to track down what the circumstances were around the particular issue.

By default the metadata of a request includes the hostname on which the issue occurred, the SCM revision on which the application is running, the request id of the request the sample was derived from. It also includes data such as the request path and the request method for web requests.


AppSignal provides two kinds of metrics.

Custom metrics allows the collection of data from just about anything. With a couple lines of code it's possible to track and graph data such as the number of registered users, visits on a page, database sizes, etc.

Host metrics is about data from the server an application is running on. Data such as CPU usage, load averages, memory usage, etc. gives more insight on performance issues than just the code itself. Maybe the disk space is running out causing the application to run much slower.

Read more about metrics on our tour page.


Namespaces are grouping mechanisms used by AppSignal to differentiate between different parts of the same application. By default AppSignal splits an application up into two namespaces: "Web" and "Background".

HTTP requests that are being monitored by AppSignal will be added to the "Web" namespace and jobs performed by background job libraries are added to the "Background" namespace. It's also possible to configure your own namespaces to differentiate between requests on an application and a private administration panel.

For more information about namespaces, please see our namespaces documentation.


AppSignal sends notifications whenever a problem is detected with an application. These notifications can be email messages, Slack messages, a PagerDuty notification, and more depending on which third-party integrations are configured.

We send notifications about errors, slow requests and Anomaly detection alerts that occur in an application using an AppSignal library. Once a problematic event is detected a notification is sent out to alert users of a problem.


Organizations are used to group together applications and users for a business. A business can have many users and clients that can be notified] whenever there's a problem with an applications.

Billing for applications is done on organization level rather than per application.

Team management is made easy using organizations. Whole teams can invited to an organization so there's no need for sharing sign-in details.

Read more about organizations in our Organizations documentation.


Organizations have people that manage the organization. These owners decide who's a member of an organization, in which team they do or do not belong and decide how the billing is done.

Organization owners can manage everything about an organization and the applications that belong to it. If you do not have permission to view or change something, ask your organization's owner to change them for you or give you more permissions.

Performance issues

Using performance monitoring it's possible to deep dive in individual requests and see what parts of an application are slow. Using this information it's possible to find and improve problem areas.

When AppSignal detects a slow web request or operation it will send an alert, because slow code can be as damaging as an error appearing.

Read more about the performance feature on our tour page.

Push API

The AppSignal "Push API" is the API endpoint used by the AppSignal agent to send the collected data to. This is different from the normal AppSignal API which is primary used to read data and add more context to the data that's sent to AppSignal.

This Push API is the API where application instrumentation is sent to from applications using the Push API key.

Push API key

The "Push API key" is a write-only API key used by applications to authenticate themselves when sending data to the AppSignal servers. Apps report data to the Push API and authenticate using this key.

There are several kinds of Push API keys that apps can use for different use cases. All Push API keys can be found on the Push & deploy settings page.

  • Organization-level Push API key:
    The recommended Push API key for back-end integrations, like Ruby, Elixir, Node.js, Python and Go. We recognize apps by a combination of the Push API key, app name and app environment, and create apps using this information if it does not yet exist.
  • App-level Push API key:
    After creation, an app also gets assigned its own app-level Push API key. This key can be used to send data to a specific app, even if the app is configured to use a different app name or app environment. This is used when moving apps between organizations without data loss, or when no app name or app environment can be set, like for the Vector integration.
  • Front-end API key:
    This key is used by apps using our front-end integration to report data and can be found in the "Front-end error monitoring" section.

Note: These write-only keys are not to be confused with the user specific API key which can be used to authenticate on the AppSignal API to retrieve data about your apps from our system.

Response time

The response time of an application's action is the time spent processing the action. The longer an action took to perform the more it qualifies as a performance issue.

The duration of this action is displayed on AppSignal.com for performance issues and in graphs for controllers/jobs and on host-level.

Ruby Magic

Did you know we write articles about the magic that is Ruby? We do!

It's called Ruby Magic and you can sign up for it on appsignal.com/ruby-magic.

A list of all the Ruby Magic articles we've written so far can be found on our blog.


When a page request is slow a hundred times in a row, it's not as relevant to see all the slow requests, a smaller sample of these requests tell the same story.

Instead the AppSignal agent only sends a small set of performance issues to the AppSignal servers. This saves data sent to the servers, the data processed on the application host and on the AppSignal servers, while still telling the same story about the issues.


Also written as "stroopwafels" in Dutch.

At AppSignal, we love stroopwafels. We've shipped over 25,000 of them to customers, friends and conferences. If you work at a tech company and have had a stroopwaffle at the office, chances it came from us.

We've written a whole article about what stroopwafels are and how you should eat them. Read on to become a stroopwaffle expert.

Now I'm hungry for a stroopwaffle.


Error and performance issue samples can be tagged. By tagging these samples it's easier to find and identify certain issues. By tagging it's possible to add more metadata to a sample that can be used in link templates to easily link to the relevant data in your own application.

Read more about tagging requests.


To manage members of organizations, organization owners can utilize teams to give access to applications. Users that are part of a team can only access the applications the team they belong to have access to. If a user is part of more than one team this user can access all applications of all the teams they belong to.

Teams give permission management to owners of an organization to limit user access to applications without the need for multiple organizations.

Read more about organization team management.

Third-party integrations

AppSignal.com provides connections with other services such as Slack and PagerDuty to more effectively alert users about problems that are detected. These integrations can be manage through the UI on AppSignal.com on an application by application basis.

Read more about which third-party integrations we offer.


Transactions are created by the AppSignal libraries for every monitored web request and background job. Within this transaction the agents monitor for errors and slow code. A transaction is created at the start of a web request or when a background job is started. Once the request or job finishes, or crashes, the transaction is closed and sent to the agent for processing.

Tags and metadata are added on transaction-level to more easily identify differences between transaction samples.


The throughput of an application is the total number of requests sent through an action/job in a certain time frame. The throughput can differ per action and per host.

On AppSignal.com the throughput is displayed per action and displayed in graphs for every host.


TLS stands for Transport Layer Security. TLS is a cryptographic protocol that provides secure connections for computer networks, for example, between a server and a web browser.

Queue time

When a server or application is busy processing a lot of requests it's possible certain requests are queued before they are processed. The time waiting to be processed is referred to as "queue time".

Since queue time can negatively affect users' experience using an application this metric is tracked by AppSignal for HTTP requests and background jobs.

For the background jobs this represents the time between request being queued (by a web request) and it being picked up by the background worker (sidekiq).

User account

Every user using AppSignal has their own user account. With this user account you can configure your personal notifications just how you want it.

Many users can belong to one or more organizations. No need for separate user accounts for every organization.

Organizations manage which users can access which applications and who can manage the billing as an owner.