What is a PAC?
A PAC (Authorized Certification Provider) is a private company authorized by Mexico's Tax Administration Service (SAT) to validate, certify and stamp Digital Tax Receipts over the Internet (CFDI) on its behalf. The figure was born with the CFDI: the SAT delegated stamping to private companies so it wouldn't receive every receipt in the country directly.
To obtain the authorization, a company must meet technical (infrastructure, security certifications, contingency), financial (capital and bonds) and operational requirements (minimum uptime, response times). The SAT publishes the official list of authorized PACs and revokes authorizations when a PAC fails to comply.
What does a PAC actually do?
When your system sends a CFDI to a PAC, the PAC executes five critical operations in seconds:
- Structural validation of the XML against the SAT's official Annex 20.
- Catalog validation: product codes, units, CFDI uses, tax regimes.
- Cross-validation with SAT services: recipient's RFC in the LCO, validity of the issuer's CSD.
- Assignment of the UUID Tax Folio and the SAT's digital seal.
- Sending a copy to the SAT and returning the stamped XML to the issuer.
The problem of depending on a single PAC
PACs are complex technical operations that suffer outages. The causes are varied: unannounced maintenance, traffic spikes, problems with SAT catalogs, infrastructure failures. When a PAC goes down, all issuers depending on it stop being able to stamp until the PAC is restored.
For a B2C issuer that invoices at point of sale (restaurants, retail, e-commerce) or a Carta Porte issuer with trucks waiting to leave, a 30-minute PAC outage translates into lost sales or detained fleets.
Additionally, the SAT can revoke a PAC's authorization. When that happens, issuers depending on that PAC have to urgently migrate to another and reconfigure their integration.
What is a multi-PAC architecture?
A multi-PAC architecture is an invoicing system that connects to several PACs simultaneously and routes each stamping to the PAC with the best availability at that moment. If a PAC fails, another takes the operation without the issuer needing to do anything.
Implementing multi-PAC well is hard because each PAC has its own API, its own authentication mechanism, its own validation quirks and its own pricing scheme. The system has to normalize responses, manage separate credentials, keep catalogs in sync with each one and, above all, decide on the spot which PAC to send each stamp to.
How automatic failover works
Failover is the technical concept of automatic switching to a backup system when the main one fails. In the multi-PAC context, it works like this:
- The system permanently monitors each PAC's health: response time, error rate, reported availability.
- It defines a primary PAC for each stamping per availability and load rules.
- If the primary PAC responds with a technical error or doesn't respond in time, the system automatically retries with a secondary PAC.
- If the secondary fails, it tries with a tertiary. Only if all fail is an error returned to the issuer.
- The issuer sees the stamped receipt normally, unaware that a switchover happened.
Benefits of operating with multi-PAC
A correctly implemented multi-PAC architecture with failover gives the issuer three operational advantages and one strategic one:
- Operational continuity: you don't stop invoicing because of a provider's outage.
- Regulatory redundancy: if the SAT revokes a PAC's authorization, you don't have to urgently migrate.
- Negotiation: a system with several PACs doesn't depend on a single provider's terms.
- Negotiation: by not depending on a single PAC, your price negotiation power improves.
How backup works at Factura Digital
At Factura Digital, stamping is ultra-fast with automatic backup, on AWS and Google Cloud infrastructure: if a PAC doesn't respond, your CFDI still gets stamped.
You invoice as usual: you enter the receipt, click stamp and get the XML and PDF. The backup works underneath, with nothing for you to do.
For high-volume, Carta Porte or B2C issuers, where an hour without invoicing is lost money, backup in stamping stops being optional.
