4. Architecture


%3 graphtik-v4.4+ flowchart cluster_compute compute operations operations compose compose operations->compose network network compose->network compile compile network->compile ●graph inputs input names inputs->compile outputs output names outputs->compile predicate node predicate predicate->compile plan execution plan compile->plan ●pruned dag execute execute plan->execute ●dag solution solution execute->solution ●solution dag values input values values->execute solution->solution ●prune dag on reschedule The definition & execution of networked operation is split in 1+2 phases:

… it is constrained by these IO data-structures:

… populates these low-level data-structures:

… and utilizes these main classes:

graphtik.fnop.FnOp(fn[, name, rescheduled, …])

An operation performing a callable (ie a function, a method, a lambda).

graphtik.pipeline.Pipeline(operations, name, *)

An operation that can compute a network-graph of operations.

graphtik.planning.Network(*operations[, graph])

A graph of operations that can compile an execution plan.

graphtik.execution.ExecutionPlan(net, needs, …)

A pre-compiled list of operation steps that can execute for the given inputs/outputs.

graphtik.execution.Solution(plan, input_values)

The solution chain-map and execution state (e.g.

… plus those for plotting:

graphtik.plot.Plotter(theme, **styles_kw)

a plotter renders diagram images of plottables.

graphtik.plot.Theme(*, _prototype, **kw)

The poor man’s css-like plot theme (see also StyleStack).


The phase where operations are constructed and grouped into pipelines and corresponding networks based on their dependencies.


combine pipelines

When operations and/or pipelines are composed together, there are two ways to combine the operations contained into the new pipeline: operation merging (default) and operation nesting.

They are selected by the nest parameter of compose() factory.

operation merging

The default method to combine pipelines, also applied when simply merging operations.

Any identically-named operations override each other, with the operations added earlier in the .compose() call (further to the left) winning over those added later (further to the right).



operation nesting

The elaborate method to combine pipelines forming clusters.

The original pipelines are preserved intact in “isolated” clusters, by prefixing the names of their operations (and optionally data) by the name of the respective original pipeline that contained them (or the user defines the renames).


Nesting, compose(), RenArgs, nest_any_node(), dep_renamed(), PlotArgs.clusters, Hierarchical data and further tricks (example).


The phase where the Network creates a new execution plan by pruning all graph nodes into a subgraph dag, and deriving the execution steps.


The phase where the plan derived from a pipeline calls the underlying functions of all operations contained in its execution steps, with inputs/outputs taken/written to the solution.

Currently there are 2 ways to execute:

  • sequential

  • (unstable) parallel, with a multiprocessing.pool.ProcessPool

Plans may abort their execution by setting the abort run global flag.


A Network.graph of operations linked by their dependencies implementing a pipeline.

During composition, the nodes of the graph are connected by repeated calls of Network._append_operation() within Network constructor.

During planning the graph is pruned based on the given inputs, outputs & node predicate to extract the dag, and it is ordered, to derive the execution steps, stored in a new plan, which is then cached on the Network class.

execution plan

Class ExecutionPlan perform the execution phase which contains the dag and the steps.

compileed execution plans are cached in Network._cached_plans across runs with (inputs, outputs, predicate) as key.


A map of dependency-named values fed to/from the pipeline during execution.

It feeds operations with inputs, collects their outputs, records the status of executed or canceled operations, tracks any overwrites, and applies any evictions, as orchestrated by the plan.

A new Solution instance is created either internally by Pipeline.compute() and populated with user-inputs, or must be created externally with those values and fed into the said method.

The class inherits collections.ChainMap, to keep the results of each operation executed in a separate solution layer dictionary (+1 for user-inputs).

The results of the last operation executed “win” in the outputs produced, and the base (least precedence) is the user-inputs given when the execution started.

Certain values may be extracted/populated with accessors.

solution layer

By default, the solution class keeps the outputs of each executed operation (and given inputs) in separate dictionaries (layers).

This layering is disabled if a jsonp dependency exists in the network, assuming that set_layered_solution() configurations has not been called with a True/False, nor has the respective parameter been given to methods compute()/execute().


Combining hierarchical data with per-operation layers in solution leads to duplications of container nodes in the data tree. To retrieve the complete solution, merging of overwritten nodes across the layers would then be needed.


solution values written by more than one operations in the respective layer, accessed by Solution.overwrites attribute (assuming that layers have not been disabled e.g. due to hierarchical data).

Note that sideffected outputs always produce an overwrite.


A subphase of planning performed by method Network._prune_graph(), which extracts a subgraph dag that does not contain any unsatisfied operations.

It topologically sorts the graph, and prunes based on given inputs, asked outputs, node predicate and operation needs & provides.

unsatisfied operation

The core of pruning & rescheduling, performed by planning.unsatisfied_operations() function, which collects all operations with unreachable dependencies:

  • they have needs that do not correspond to any of the given inputs or the intermediately computed outputs of the solution;

  • all their provides are NOT needed by any other operation, nor are asked as outputs.

execution dag
solution dag

There are 2 directed-acyclic-graphs instances used:

execution steps

The plan contains a list of the operation-nodes only from the dag, topologically sorted, and interspersed with instruction steps needed to compute the asked outputs from the given inputs.

They are built by Network._build_execution_steps() based on the subgraph dag.

The only instruction step other than an operation is for performing an eviction.


A memory footprint optimization where intermediate inputs & outputs are erased from solution as soon as they are not needed further down the dag.

Evictions are pre-calculated during planning, denoted with the dependency inserted in the steps of the execution plan.


The named input values that are fed into an operation (or pipeline) through Operation.compute() method according to its needs.

These values are either:


The dictionary of computed values returned by an operation (or a pipeline) matching its provides, when method Operation.compute() is called.

Those values are either:

  • retained in the solution, internally during execution, keyed by the respective provide, or

  • returned to user after the outer pipeline has finished computation.

When no specific outputs requested from a pipeline, Pipeline.compute() returns all intermediate inputs along with the outputs, that is, no evictions happens.

An operation may return partial outputs.


The Pipeline composes and computes a network of operations against given inputs & outputs.

This class is also an operation, so it specifies needs & provides but these are not fixed, in the sense that Pipeline.compute() can potentially consume and provide different subsets of inputs/outputs.


Either the abstract notion of an action with specified needs and provides, dependencies, or the concrete wrapper FnOp for (any callable()), that feeds on inputs and update outputs, from/to solution, or given-by/returned-to the user by a pipeline.

The distinction between needs/provides and inputs/outputs is akin to function parameters and arguments during define-time and run-time, respectively.


The (possibly hierarchical) name of a solution value an operation needs or provides.

  • Dependencies are declared during composition, when building FnOp instances. Operations are then interlinked together, by matching the needs & provides of all operations contained in a pipeline.

  • During planning the graph is then pruned based on the reachability of the dependencies.

  • During execution Operation.compute() performs 2 “matchings”:

    • inputs & outputs in solution are accessed by the needs & provides names of the operations;

    • operation needs & provides are zipped against the underlying function’s arguments and results.

    These matchings are affected by modifiers, print-out with diacritics.

Differences between various dependency operation attributes:

dependency attribute




















The list of dependency names an operation requires from solution as inputs,

roughly corresponding to underlying function’s arguments (fn_needs).

Specifically, Operation.compute() extracts input values from solution by these names, and matches them against function arguments, mostly by their positional order. Whenever this matching is not 1-to-1, and function-arguments differ from the regular needs, modifiers must be used.


The list of dependency names an operation writes to the solution as outputs,

roughly corresponding to underlying function’s results (fn_provides).

Specifically, Operation.compute() “zips” this list-of-names with the output values produced when the operation’s function is called. Whenever this “zipping” is not 1-to-1, and function-results differ from the regular operation (op_provides) (or results are not a list), it is possible to:


Map an existing name in fn_provides into a duplicate, artificial one in op_provides .

You cannot alias an alias. See Aliased provides

conveyor operation
default identity function

The default function if none given to an operation that conveys needs to provides.

For this to happen when FnOp.compute() is called, an operation name must have been given AND the number of provides must match that of the number of needs.


Default conveyor operation & identity_function().

returns dictionary

When an operation is marked with FnOp.returns_dict flag, the underlying function is not expected to return fn_provides as a sequence but as a dictionary; hence, no “zipping” of function-results –> fn_provides takes place.

Usefull for operations returning partial outputs to have full control over which outputs were actually produced, or to cancel sideffects.


A modifier change dependency behavior during planning or execution.

For instance, a needs may be annotated as keyword() and/or optionals function arguments, provides and needs can be annotated as “ghost” sideffects or assigned an accessor to work with hierarchical data.

The representation of modifier-annotated dependencies utilize a combination of these diacritics:

>   : keyword()
?   : optional()
*   : vararg()
+   : varargs()
$   : accessor (mostly for jsonp)

See graphtik.modifier module.


A needs only modifier for a inputs that do not hinder operation execution (prune) if absent from solution.

In the underlying function it corresponds to either:


A needs only modifier for inputs to be appended as *args (if present in solution).

There are 2 kinds, both, by definition, optionals:

  • the vararg() annotates any solution value to be appended once in the *args;

  • the varargs() annotates iterable values and all its items are appended in the *args one-by-one.


To avoid user mistakes, varargs do not accept str inputs (though iterables):

>>> graph(a=5, b="mistake")
Traceback (most recent call last):
ValueError: Failed preparing needs:
    1. Expected needs['b'(+)] to be non-str iterables!
    +++inputs: ['a', 'b']
                           needs=['a', 'b'(+)],

In printouts, it is denoted either with * or + diacritic.

See also the elaborate example in Hierarchical data and further tricks section.


A modifier denoting a fictive dependency linking operations into virtual flows, without real data exchanges.

The side-effect modification may happen to some internal state not fully represented in the graph & solution.

There are actually 2 relevant modifiers:

  • An abstract sideffect modifier (annotated with sfx()) describing modifications taking place beyond the scope of the solution. It may have just the “optional” diacritic in printouts.

  • The sideffected modifier (annotated with sfxed()) denoting modifications on a real dependency read from and written to the solution.

Both kinds of sideffects participate in the planning of the graph, and both may be given or asked in the inputs & outputs of a pipeline, but they are never given to functions. A function of a returns dictionary operation can return a falsy value to declare it as canceled.


A modifier that denotes sideffects on a dependency that exists in solution, allowing to declare an operation that both needs and provides that sideffected dependency.


To be precise, the “sideffected dependency” is the name held in _Modifier.sideffected attribute of a modifier created by sfxed() function.

The outputs of a sideffected dependency will produce an overwrite if the sideffected dependency is declared both as needs and provides of some operation.

It is annotated with sfxed(); it may have all diacritics in printouts.

See also the elaborate example in Hierarchical data and further tricks section.


Getter/setter functions to extract/populate solution values given as a modifier parameter (not applicable for pure sideffects).

See Accessor defining class and the jsonp() concrete factory.

doc chain
hierarchical data

A subdoc is a dependency value nested further into another one (the superdoc), accessed with a json pointer path expression with respect to the solution, denoted with slashes like: root/parent/child/leaf

Note that if a nested output is asked, then all docs-in-chain are kept i.e. all superdocs till the root dependency (the “superdocs”) plus all its subdocs (the “subdocs”); as depicted below for a hypothetical dependency /stats/b/b1:

digraph { rankdir=LR; stats -> a -> {a1, a2} [color=grey] stats -> b -> b1 -> {b11, b12, b13} b13 -> b131 stats -> c -> {c1, c2} [color=grey] b1 [fontname=bold penwidth=3] a [color=grey fontcolor=grey] a1 [color=grey fontcolor=grey label="..."] a2 [color=grey fontcolor=grey label="..."] c [color=grey fontcolor=grey] c1 [color=grey fontcolor=grey label="..."] c2 [color=grey fontcolor=grey label="..."] }

For instance, if the root has been asked as output, no subdoc can be subsequently evicted.


:Hierarchical data and further tricks (example)

json pointer path

A modifier containing slashes(/) accessing subdoc values with json pointer expressions, like root/parent/child/1/item.

The first step (e.g. root) is the name of a dependency in the solution which becomes the root document for the jsonp expression following.

partial outputs
canceled operation

The partial pruning of the solution’s dag during execution. It happens when any of these 2 conditions apply:

the solution must then reschedule the remaining operations downstream, and possibly cancel some of those ( assigned in Solution.canceled).

Partial operations are usually declared with returns dictionary so that the underlying function can control which of the outputs are returned.

See Operations with partial outputs (rescheduled)


Keep executing as many operations as possible, even if some of them fail. Endurance for an operation is enabled if set_endure_operations() is true globally in the configurations or if FnOp.endured is true.

You may interrogate Solution.executed to discover the status of each executed operations or call one of check_if_incomplete() or scream_if_incomplete().

See Depending on sideffects

node predicate

A callable(op, node-data) that should return true for nodes to be included in graph during planning.

abort run

A global configurations flag that when set with abort_run() function, it halts the execution of all currently or future plans.

It is reset automatically on every call of Pipeline.compute() (after a successful intermediate planning), or manually, by calling reset_abort().

parallel execution
execution pool

execute operations in parallel, with a thread pool or process pool (instead of sequential). Operations and pipeline are marked as such on construction, or enabled globally from configurations.

Note a sideffects are not expected to function with process pools, certainly not when marshalling is enabled.

process pool

When the multiprocessing.pool.Pool class is used for parallel execution, the tasks must be communicated to/from the worker process, which requires pickling, and that may fail. With pickling failures you may try marshalling with dill library, and see if that helps.

Note that sideffects are not expected to function at all. certainly not when marshalling is enabled.

thread pool

When the multiprocessing.dummy.Pool() class is used for parallel execution, the tasks are run in process, so no marshalling is needed.


Pickling parallel operations and their inputs/outputs using the dill module. It is configured either globally with set_marshal_tasks() or set with a flag on each operation / pipeline.

Note that sideffects do not work when this is enabled.


Objects that can plot their graph network, such as those inheriting Plottable, (FnOp, Pipeline, Network, ExecutionPlan, Solution) or a pydot.Dot instance (the result of the Plottable.plot() method).

Such objects may render as SVG in Jupiter notebooks (through their plot() method) and can render in a Sphinx site with with the graphtik RsT directive. You may control the rendered image as explained in the tip of the Plotting section.

SVGs are in rendered with the zoom-and-pan javascript library


Zoom-and-pan does not work in Sphinx sites for Chrome locally - serve the HTML files through some HTTP server, e.g. launch this command to view the site of this project:

python -m http.server 8080 --directory build/sphinx/html/

A Plotter is responsible for rendering plottables as images. It is the active plotter that does that, unless overridden in a Plottable.plot() call. Plotters can be customized by various means, such plot theme.

active plotter
default active plotter

The plotter currently installed “in-context” of the respective graphtik configuration - this term implies also any Plot customizations done on the active plotter (such as plot theme).

Installation happens by calling one of active_plotter_plugged() or set_active_plotter() functions.

The default active plotter is the plotter instance that this project comes pre-configured with, ie, when no plot-customizations have yet happened.


It is recommended to use other means for Plot customizations instead of modifying directly theme’s class-attributes.

All Theme class-attributes are deep-copied when constructing new instances, to avoid modifications by mistake, while attempting to update instance-attributes instead (hint: allmost all its attributes are containers i.e. dicts). Therefore any class-attributes modification will be ignored, until a new Theme instance from the patched class is used .

plot theme
current theme

The mergeable and expandable styles contained in a plot.Theme instance.

The current theme in-use is the Plotter.default_theme attribute of the active plotter, unless overridden with the theme parameter when calling Plottable.plot() (conveyed internally as the value of the PlotArgs.theme attribute).

style expansion

A style is an attribute of a plot theme, either a scalar value or a dictionary.

Styles are collected in stacks and are merged into a single dictionary after performing the following expansions:

  • Resolve any Ref instances, first against the current nx_attrs and then against the attributes of the current theme.

  • Render jinja2 templates (see _expand_styles()) with template-arguments all the attributes of the plot_args instance in use (hence much more flexible than Ref).

  • Call any callables with current plot_args and replace them by their result (even more flexible than templates).

  • Any Nones results above are discarded.

  • Workaround pydot/pydot#228 pydot-cstor not supporting styles-as-lists.


if DEBUG is enabled, the provenance of all style values appears in the tooltips of plotted graphs.

graphtik configuration

The functions controlling compile & execution globally are defined in config module and +1 in graphtik.plot module; the underlying global data are stored in contextvars.ContextVar instances, to allow for nested control.

All boolean configuration flags are tri-state (None, False, True), allowing to “force” all operations, when they are not set to the None value. All of them default to None (false).


When operations fail, the original exception gets annotated with salvaged values from locals() and raised intact.

See Jetsam on exceptions.