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Abstract:

This talk, presented by two speakers, delves into effective patterns and tools for orchestrating complex operations and handling side effects within Elixir/Ecto web applications. The first speaker outlines a layered architectural approach for web applications, emphasizing where different concerns like error handling should reside. They introduce strategies for managing tasks based on their required success outcome: "must happen now," "must happen eventually," and "fire and forget." Ecto Multi is proposed for atomic "must happen now" operations, while Oban is recommended for resilient "must happen eventually" jobs. Transaction callbacks, executed only on transaction commit, are suggested for "fire and forget" tasks like logging and metrics. A strong warning is issued against using Ecto Multi's run function for arbitrary logic, particularly side effects that cannot be rolled back, which can lead to misleading states and logs. The concept of composing complex operations by having functions return Ecto Multi objects (create_multis) is highlighted for improved separation of concerns and code consistency. The second speaker then introduces Ecto window functions as a flexible SQL feature for performing calculations on subsets of rows (windows) while returning individual rows, contrasting them with aggregate functions and detailing their use within Ecto queries via the OVER clause components (PARTITION BY, ORDER BY, frame clause).

Navigating Complex Operations and Data with Elixir/Ecto Patterns

• 0:26 Application Layers: The speaker outlines a mental model for web application layers (endpoint -> web layer -> business logic -> database) to guide where different behaviors and error handling should be placed for consistency.
• 0:56 Consistent Error Handling: A key pattern suggests letting the web layer handle HTTP status codes, while deeper layers return generic errors that the web layer translates, leading to a more uniform codebase.
• 1:35 Orchestration Types: Two types of orchestration are defined: 'Traffic Cop' (directing requests, validation) and 'Coordinating Work' (performing actual data manipulation and side effects). The talk focuses on tools for the latter.
• 2:30 Requirement Buckets: Requirements for work are categorized into three types based on success criteria: "Must be successful now" (fails request if fails), "Must be successful eventually" (doesn't fail request but needs retries/guarantee), and "Fire and forget" (logging, metrics; assumed stable, no error handling needed).
• 3:21 Tools for Requirements: Ecto Multi, Oban, and Transaction Callbacks are introduced as tools tailored to handle these three types of requirements respectively.
• 3:33 Ecto Multi Overview: Ecto Multi is explained as a structure to group multiple Repo operations that execute atomically within a single transaction. It's emphasized that Multis are opaque and require using the official Multi API functions.
• 4:40 Unique Operation Names: Operations added to an Ecto Multi must have unique names to avoid collisions, as they are stored internally similar to a map set.
• 6:34 Caution Against Ecto.Multi.run: The speaker strongly advises against using Multi.run for arbitrary functions, calling it a "foot gun," because it can introduce nested transactions and execute side effects (like API calls or logging) that cannot be rolled back if the main transaction fails, leading to inconsistent states and misleading information.
• 8:03 Transaction Callbacks: Introduced as a mechanism to execute logic after an Ecto transaction has successfully committed. These are suitable for "fire and forget" tasks like logging or metrics, as they only run on success.
• 8:50 Callback Implementation: Callbacks are added to the Multi as key-value pairs where the value is a function. They require unique, namespaced names and rely on the assumption that the main transaction is not nested (avoiding the problematic Multi.run).
• 12:52 Oban for Eventual Success: Oban, an Elixir job queue, is presented as the ideal tool for "Must be successful eventually" tasks (like sending emails) due to its built-in resilience (retries), worker processing, and monitoring capabilities.
• 13:37 Transactional Job Insertion: Oban supports multi_insert, allowing jobs to be added to the job queue transactionally. If the main Ecto transaction rolls back, the Oban job insertion is also rolled back.
• 16:17 Composing Multis: A recommended pattern is to create functions (e.g., create_multis) within context modules that return a Multi object containing all operations and side effects related to that context's action (e.g., creating a user). This allows other modules to compose complex operations by merging these Multis without needing to know their internal details, promoting separation of concerns and reducing boilerplate.
• 18:19 Predictable Outcomes: Using these patterns ensures that when a complex operation succeeds, you know exactly what happened (all atomic ops committed, all Oban jobs scheduled, all callbacks executed), and when it fails, you know exactly what didn't happen (main transaction rolled back, no Oban jobs scheduled, no callbacks ran), making debugging much easier.
• 21:31 Problem with Multi.run and Side Effects: Re-demonstrates how using Multi.run to call functions containing side effects (like logging) can lead to metrics being emitted even if the main transaction ultimately fails, resulting in logs/metrics that "lie" about the system state.
• 23:18 Trustworthy Metrics: Transaction callbacks, when used correctly (without nested transactions via Multi.run), only execute post-commit, guaranteeing that metrics and logs generated within them reflect actions that actually persisted to the database.
• 23:47 Benefits of Patterns: Adopting strong patterns reduces developer decision fatigue, increases code consistency and readability, establishes a common team vocabulary, and provides a framework for making sound design choices.
• 27:11 Ecto Window Functions: Second speaker (Melissa) introduces SQL window functions as a powerful Ecto feature.
• 27:55 Window Function Definition: Window functions perform calculations over a defined set of rows (a "window") but return the computed value alongside each individual row, unlike aggregate functions (GROUP BY) which collapse rows. Useful for calculating percentages, running totals, or comparing rows within a group without needing a join.
• 29:25 SQL and Ecto Syntax: Explains the SQL OVER clause (function() OVER (PARTITION BY ... ORDER BY ... frame ...)) and demonstrates the corresponding Ecto syntax using over/2, partition_by/2, order_by/2, etc.
• 29:48 OVER Clause Components: Details the components of the OVER clause: empty OVER() for the whole table, PARTITION BY to divide rows into groups, ORDER BY to sort within partitions, and the frame clause (RANGE, ROWS, GROUPS) for specifying a subset of rows within a partition for calculation.
• 32:21 Encouragement: Concludes by encouraging the use of built-in Ecto functions for implementing window functions.