Concepts and Organisation

Object types

Conceptually, TPV consists of the following types of objects.

1. Entities - An entity is anything that will be considered for scheduling by TPV. Entities include Tools, Users, Groups, Rules and Destinations. All entities have some common properties (id, cores, mem, env, params, and scheduling tags).

2. Scheduling Tags - Entities can have scheduling tags defined on them that determine which entities match up, and which destination they can schedule on. Tags fall into one of four categories, (required, preferred, accepted, rejected), ranging from indicating a requirement for a particular destination, to indicating complete aversion.

3. Loader - The loader is responsible for loading entity definitions from a config file. The loader will parse and validate entity definitions, including compiling python expressions, and processing inheritance, to produce a list of entities suitable for mapping. The loader is also capable of loading config files from multiple sources, including https urls.

4. Mapper - The mapper is responsible for routing a Galaxy job to its destination, based on the current user, tool and job that must be scheduled. The mapper will respect the scheduling constraints expressed by the loaded entities.

Operations

When a mapper routes jobs to a destination, it does so by applying 5 basic operations on entities.

1. Inherit

The inherit operation enables an entity to inherit the properties of another entity of the same type, and to override any required properties. While a Tool can inherit another tool, which can in-turn inherit yet another tool, it cannot inherit a User, as it is a different entity type. It is also possibly to globally define a default_inherits field, which is the entity that all entities will inherit from should they not have an inherits tag explicitly defined. Inheritance is generally processed at load time by the Loader, so that there is no cost at runtime. However, the Mapper will process default inheritance, should the user, role or tool that is being dispatched not have an entry in the entities list.

When inheriting scheduling tags, if the same tag is defined by both the parent and the child, only the child’s tag will take effect. For example, if a parent defines high-mem as a required tag, but a child defines high-mem as a preferred tag, then the tag will be treated as a preferred tag.

2. Combine

The combine operation matches up the current user, role and tool entities, and creates a combined entity that shares all their respective preferences. If the same property is defined on both entities, the entity with the higher merge priority will override the other. The priority order is fixed in the following way: Destination > User > Role > Tool. For example, if a tool specifies cores, and a user also specifies cores, the user’s cores value will take precedence. Properties defined on destinations have the highest priority of all. The combine operation follows the following additional rules:

Combining scheduling tags

When combining scheduling tags, if a role expresses a preferences for tag training for example, and a tool expresses a requirement for tag high-mem, the combined entity would share both preferences. This can be used to route certain roles or users to specific destinations for example.

However, if the tags are mutually exclusive, then an IncompatibleTagsException is raised. For example, if a role expresses a preference for training, but the tool rejects tag training, then the job can no longer be scheduled. If the tags are compatible, then the tag with the stronger claim takes effect. For example, if a tool requires high-mem and a user prefers high-mem, then the combined entity will require high-mem. An example of using this property would be to restrict the availability of dangerous tools only to trusted users.

Combining envs and params

In this case, these requirements are simply merged, with duplicate envs and params merged in the following order: Destination > User > Role > Tool.

3. Evaluate

This operation evaluates any python expressions in the combined entity. At this point, rules are also evaluated. After evaluation, expressions such as cores, mem, max_cores, min_gpus etc., will all have concrete values. During evaluation, the cores, mem and gpu values are clamped between min_cores, min_mem, min_gpus and max_cores, max_mem, max_gpus. Afterwards, these values can be compared with a destination’s values, as described in the match step next.

4. Match

The match operation is used to find matching destinations for the combined, evaluated entity. This step ensures that the destination has sufficient gpus, cores and mem to satisfy the entity’s request. The maximum size of a job that a destination can accept can be defined using the max_accepted_cores, max_accepted_mem and max_accepted_gpus fields. If these are not defined, a match is assumed. In addition, all destinations that do not have scheduling tags required by the entity are rejected, and all destinations that have scheduling tags rejected by the entity are also rejected. Preference and acceptance is not considered at this stage, simply compatibility with available destination based on the tag compatibility table documented later.

5. Rank

After the matching destinations are short listed, they are ranked using a pluggable rank function. The default rank function simply sorts the destinations by tags that have the most number of preferred tags, with a penalty if preferred tags are absent. However, this default rank function can be overridden per entity, allowing a custom rank function to be defined in python code, with arbitrary logic for picking the best match from the available candidate destinations.

Job Dispatch Process

When a typical job is dispatched, TPV follows the process below.

1. lookup - Looks up Tool, User and Role entity definitions that match the job.

2. combine() - Combines entity requirements to create a merged entity.

3. evaluate() - Evaluates expressions in combined entity.

4. match() - Matches the combined entity requirements with a suitable destination.

5. rank() - Rank the matching destinations using a pluggable rank function.

6. choose - The entity is combined with the best matching destination and any expressions on the destination are evaluated, with the first non-failing match chosen (no rule failures).

Expressions

Most TPV properties can be expressed as Python expressions. The rule of thumb is that all string expressions are evaluated as python f-strings, and all integers or boolean expressions are evaluated as python code blocks. For example, cpu, cores and mem are evaluated as python code blocks, as they evaluate to integer/float values. However, env and params are evaluated as f-strings, as they result in string values. This is to improve the readability and syntactic simplicity of TPV config files.

At the point of evaluating these functions, there is an evaluation context, which is a default set of variables that are available to that expression. The following default variables are available to all expressions:

Default evaluation context

Variable	Description
app	the Galaxy App object
tool	the Galaxy tool object
user	the current Galaxy user object
job	the Galaxy job object
mapper	the TPV mapper object, which can be used to access parsed TPV configs
entity	the TPV entity being currently evaluated. Can be a combined entity.
self	an alias for the current TPV entity.

Custom evaluation contexts

These are user defined context values that can be defined globally, or locally at the level of each entity. Any defined context value is available as a regular variable at the time the entity is evaluated.

Special evaluation contexts

In addition to the defaults above, additional context variables are available at different steps.

gpu, core and mem expressions - these are evaluated in order, and thus can be referred to in that same order. For example, gpu expressions cannot refer to core and mem, as they have not been evaluated yet. cpu expressions can be based on gpu values. mem expressions can refer to both cores and gpus.

env and param expressions - env expressions can be based on gpu, cores or mem. param expressions can additional refer to evaluated env expressions.

rank functions - these can refer to all prior expressions, and are additional passed in a candidate_destinations array, which is a list of matching TPV destinations.

Properties that do not support expressions

Some properties do not support expressions. These are primarily:

• max_accepted_cores, max_accepted_mem and max_accepted_gpus, which can only be defined on destinations. This is because when a combined entity is matched with a destination, concrete values are required.

• tags defined on entities

Scheduling

TPV offers several mechanisms for controlling scheduling, all of which are optional. In its simplest form, no scheduling constraints would be defined at all, in which case the entity would schedule on the first available destination. Admins can use scheduling tags to exert additional control over which destinations jobs can schedule. Scheduling tags fall into one of four categories, (required, preferred, accepted, rejected), ranging from indicating a requirement for a particular entity, to indicating complete aversion.

Tag Type	Description
require	required tags must match up for scheduling to occur. For example, if a tool is marked as requiring the high-mem tag, only destinations that are tagged as requiring, preferring or accepting the high-mem tag would be considering for scheduling.
prefer	prefer tags are ranked higher that accept tags when scheduling decisions are made.
accept	accept tags can be used to indicate that a entity can match up or support another entity, even if not preferentially.
reject	reject tags cannot be present for scheduling to occur. For example, if a tool is marked as rejecting the pulsar tag, only destinations that do not have that tag are considered for scheduling. If two entities have the same reject tag, they still repel each other.

Scheduling tag compatibility table

Tag Type	Require	Prefer	Accept	Reject	Not Tagged
Require	✓	✓	✓	✕	✕
Prefer	✓	✓	✓	✕	✓
Accept	✓	✓	✓	✕	✓
Reject	✕	✕	✕	✕	✓
Not Tagged	✕	✓	✓	✓	✓

Scheduling by tag match

Scheduling tags can be used to model anything from compatibility with a destination, to permissions to execute a tool. (e.g. a tool can be tagged as requiring the “restricted” tag, and users can be tagged as rejecting the “restricted” tag by default. Then, only users who are specifically marked as requiring, tolerating, or preferring the “restricted” tag can execute that tool. Of course, the destination must also be marked as not rejecting the “restricted” tag.

Scheduling by rules

Rules can be used to conditionally modify any entity requirement. Rules can be given an ID, which can subsequently be used by an inheriting entity to override the rule. If no ID is specified, a unique ID is generated, and the rule can no longer be overridden. Rules are typically evaluated through an if clause, which specifies the logical condition under which the rule matches. If the rule matches, cores, memory, scheduling tags etc. can be specified to override inherited values. The special clause fail can be used to immediately fail the job with an error message. The execute clause can be used to execute an arbitrary code block on rule match.

Scheduling by custom ranking functions

The default rank function sorts destinations by scoring how well the tags match the job’s requirements. As this may often be too simplistic, the rank function can be overridden by specifying a custom rank clause. The rank clause can contain an arbitrary code block, which can do the desired sorting, for example by determining destination load by querying the job manager, influx statistics etc. The final statement in the rank clause must be the list of sorted destinations.