Report PDFs in Phoenix

Why reports become PDFs

A report is a snapshot of numbers at a moment in time. Its whole value is that it does not change when the underlying dashboard does. That is the difference between a document and a view of a database, and it is why analytics, operations and account teams keep asking for this.

What a report has to carry

Before any of the code below matters, the template has to produce a document that is actually a report. These are the fields that make it one, and the ones a reader or an auditor will look for first.

• the reporting period as an explicit start and end date
• the generation timestamp, distinct from the period
• the data source or filter set, so a reader can tell what was excluded
• each metric with its unit, since a bare number is not a figure
• comparison against the prior period where the report claims a trend

Turning your HEEx template into a document

You already have the markup. Phoenix.Template.render_to_string(MyAppWeb.PDFHTML, "invoice", "html", assigns) gives you it as a string, which is the only input the render needs. A HEEx template that references ~p sigil paths produces paths relative to your endpoint. Use Endpoint.url() to build absolute ones, or inline the stylesheet.

The Phoenix implementation

Req is the client most projects here already have. The render happens in a controller action using send_download, and you send_download with a binary, or chunk through Plug.Conn.

# lib/my_app_web/controllers/report_controller.ex
defmodule MyAppWeb.ReportController do
  use MyAppWeb, :controller

  def pdf(conn, %{"id" => id}) do
    report = MyApp.Reports.get!(id)

    html =
      Phoenix.Template.render_to_string(
        MyAppWeb.PDFHTML,
        "report",
        "html",
        report: report
      )

    # 60 seconds, not the default. The renders that time out are exactly the
    # documents that were doing real work.
    %{status: 200, body: pdf} =
      Req.post!("https://api.pdfpipe.xyz/v1/pdf",
        auth: {:bearer, Application.fetch_env!(:my_app, :pdfpipe_key)},
        json: %{html: html, options: %{format: "A4", printBackground: true}},
        receive_timeout: 60_000
      )

    send_download(conn, {:binary, pdf},
      filename: "report-#{id}.pdf",
      content_type: "application/pdf"
    )
  end
end

The CSS that makes a report page correctly

The layout problem specific to this document is that charts and tables must not split mid-element, and a report long enough to matter will page-break somewhere awkward unless break rules are set. These rules handle it.

/* Charts and tables must not split. Sections start on a fresh
   page so a heading never ends up alone at the foot of one. */
.chart, figure, table { break-inside: avoid; }
h2 { break-after: avoid; }
section { break-before: page; }
section:first-of-type { break-before: auto; }

@page { size: A4; margin: 20mm 18mm; }

What goes wrong in Phoenix

The default receive timeout on the HTTP client is shorter than a heavy render takes, so the requests that fail are exactly the documents that were doing real work. Raise it explicitly for this call rather than globally.

What people try first

Most Phoenix projects reach for ChromicPDF, which supervises an external Chrome process inside your own supervision tree, making the browser's lifecycle part of your application's lifecycle. That works until it is running on more than one machine, at which point the browser becomes the thing you operate rather than the thing you use.

Where the report lives afterwards

Rendering is the short part. Reports pile up faster than any other document here because they are produced on a schedule whether or not anyone reads them. Most teams keep twelve months and expire the rest, which makes storage lifecycle a design decision rather than an afterthought.

Getting the document right

• Check the period the figures cover, because a report without an unambiguous date range is worse than no report.
• Generation is triggered by a schedule, usually month end or week end, so size the timeout for that path rather than for a health check.
• This is a batch shape: render the run through the batch endpoint rather than firing thousands of individual requests.
• Backgrounds are painted by default here, so a design that uses colour needs nothing set. Only an explicit print_background of false turns them off.

Frequently asked

Related

Other Phoenix documents, and the same report in other stacks.